ramips: Add support for Mikrotik RouterBOARD RBM33g

ramips: Add support for Mikrotik RouterBOARD RBM33g

This commit adds support for the Mikrotik RouterBOARD RBM33g.

=Hardware=

The RBM33g is a mt7621 based device featuring three gigabit ports, 2
miniPCIe slots with sim card sockets, 1 M.2 slot, 1 USB 3.0 port and a male
onboard RS-232 serial port. Additionally there are a lot of accessible
GPIO ports and additional buses like i2c, mdio, spi and uart.

==Switch==

The three Ethernet ports are all connected to the internal switch of the
mt7621 SoC:

port 0: Ethernet Port next to barrel jack with PoE printed on it
port 1: Innermost Ethernet Port on opposite side of RS-232 port
port 2: Outermost Ethernet Port on opposite side of RS-232 port
port 6: CPU

==Flash==

The device has two spi flash chips. The first flash chips is rather small
(512 kB), connected to CS0 by default and contains only the RouterBOOT
bootloader and some factory information (e.g. mac address).
The second chip has a size of 16 MB, is by default connected to CS1 and
contains the firmware image.

==PCIe==

The board features three PCIe-enabled slots. Two of them are miniPCIe
slots (PCIe0, PCIe1) and one is a M.2 (Key M) slot (PCIe2).
Each of the miniPCIe slots is connected to a dedicated mini SIM socket
on the back of the board.

Power to all three PCIe-enabled slots is controlled via GPIOs on the
mt7621 SoC:

PCIe0: GPIO9
PCIe1: GPIO10
PCIe2: GPIO11

==USB==

The board has one external USB 3.0 port at the rear. Additionally PCIe
port 0 has a permanently enabled USB interface. PCIe slot 1 shares its
USB interface with the rear USB port. Thus only either the rear USB port
or the USB interface of PCIe slot 1 can be active at the same time. The
jumper next to the rear USB port controls which one is active:

open: USB on PCIe 1 is active
closed: USB on rear USB port is active

==Power==

The board can accept both, passive PoE and external power via a 2.1 mm
barrel jack. The input voltage range is 11-32 V.

=Installation=

==Prerequisites==

A USB -> RS-232 Adapter and a null modem cable are required for
installation.

To install an OpenWRT image to the device two components must be built:

1. A openwrt initramfs image
2. A openwrt sysupgrade image

===initramfs & sysupgrade image===

Select target devices "Mikrotik RBM33G" in
openwrt menuconfig and build the images. This will create the images
"openwrt-ramips-mt7621-mikrotik_rbm33g-initramfs-kernel.bin" and
"openwrt-ramips-mt7621-mikrotik_rbm33g-squashfs-sysupgrade.bin" in the output
directory.

==Installing==

**Make sure to back up your RouterOS license in case you do ever want to
go back to RouterOS using "/system license output" and back up the created
license file.**

Serial settings: 115200 8N1

The installation is a two-step process. First the
"openwrt-ramips-mt7621-mikrotik_rbm33g-initramfs-kernel.bin" must be booted
via tftp:

1. Set up a dhcp server that points the bootfile to tftp server serving
   the "openwrt-ramips-mt7621-mikrotik_rbm33g-initramfs-kernel.bin"
   initramfs image
2. Connect to WAN port (left side, next to sys-LED and power indicator)
3. Connect to serial port of board
4. Power on board and enter RouterBOOT setup menu
5. Set boot device to "boot over ethernet"
6. Set boot protocol to "dhcp protocol" (can be omitted if DHCP server
   allows dynamic bootp)
6. Save config
7. Wait for board to boot via Ethernet

On the serial port you should now be presented with the OpenWRT boot log.
The next steps will install OpenWRT persistently.

1. Copy "openwrt-ramips-mt7621-mikrotik_rbm33g-squashfs-sysupgrade.bin" to the device
   using scp.
2. Write openwrt to flash using "sysupgrade
   openwrt-ramips-mt7621-mikrotik_rbm33g-squashfs-sysupgrade.bin"

Once the flashing completes reboot the router and let it boot from flash.
It should boot straight to OpenWRT.

Signed-off-by: Tobias Schramm <tobleminer@gmail.com>