ar71xx: add support for Mikrotik RB750P-PBr2

ar71xx: add support for Mikrotik RB750P-PBr2

Specifications:
- SoC: Qualcomm QCA9531 (650MHz)
- RAM: 64MB
- Storage: 16MB NOR SPI flash
- Ethernet: 5x100M (1 PoE in, 4 PoE out)
- Outdoor use ready

This ethernet router is based on the same platform as the hEX PoE lite.

Installation

1. login to the Mikrotik WebUI to backup your licence keys
2. setup a DHCP/BOOTP Server with:
     * DHCP-Option 66 (TFTP server name) pointing to a local TFTP
       Server within the same subnet of the DHCP range
     * DHCP-Option 67 (Bootfile-Name) matching the initramfs filename
       of the to be booted image
3. connect the port labled internet to your local network
4. keep the reset button pushed down and power on the board

The board should load and start the initramfs image from the TFTP
Server. Login as root/without password to the started LEDE via ssh
listing on IPv4 address 192.168.1.1. Use sysupgrade to install LEDE.

Revert to RouterOS

Use the "rbcfg" package on in LEDE:
  * rbcfg set boot_protocol bootp
  * rbcfg set boot_device ethnand
  * rbcfg apply

Open Netinstall and reboot routerboard. Now netinstall sees routerboard
and you can install RouterOS. If NetInstall gets stuck on Sending offer
just wait for it to timeout and then close and open Netinstall again.

Click on install again.

In order for RouterOS to function properly, you need to restore license
for the device. You can do that by including license in NetInstall

Signed-off-by: Robert Marko <robimarko@gmail.com>