ramips: add support for MikroTik RouterBOARD 760iGS (hEX S)

ramips: add support for MikroTik RouterBOARD 760iGS (hEX S)

This patch adds support for the MikroTik RouterBOARD 760iGS router.
It is similar to the already supported RouterBOARD 750Gr3.
The 760iGS device features an added SFP cage, and passive
PoE out on port 5 compared to the RB750Gr3.

https://mikrotik.com/product/hex_s

Specifications:

- SoC: MediaTek MT7621A
- CPU: 880MHz
- Flash: 16 MB
- RAM:  256 MB
- Ethernet: 5x 10/100/1000 Mbps
- SFP cage
- USB port
- microSD slot

Unsupported:

- Beeper (requires PWM driver)
- ZT2046Q (ADS7846 compatible) on SPI as slave 1 (CS1)
  The linux driver requires an interrupt, and pendown GPIO
  These are unknown, and not needed with the touchscreen
  only used for temperature and voltage monitoring.
  ads7846 hwmon:
  temp0 is degrees Celsius
  temp1 is voltage * 32

GPIOs:

- 07:  input passive PoE out (lan5) compatible (Mikrotik) device connected
- 17:  output passive PoE out (lan5) switch

Installation through RouterBoot follows the usual MikroTik method
https://openwrt.org/toh/mikrotik/common

To boot to intramfs image in RAM:

1. Setup TFTP server to serve intramfs image.
2. Plug Ethernet cable into WAN port.
3. Unplug power, hold reset button and plug power in.
   Wait (~25 seconds) for beep and then release reset button.
   The SFP LED will be lit in RouterBoot, but will not be lit in OpenWRT.
4. Wait for a minute. Router should be running OpenWrt,
   check by plugging in to port 2-5 and going to 192.168.1.1.

To install OpenWrt to flash:

1. Follow steps above to boot intramfs image in RAM.
2. Flash the sysupgrade.bin image with web interface or sysupgrade.
3. Once the router reboots you will be running OpenWrt from flash.

OEM firmware differences:

- RouterOS assigns a different MAC address for each port
- The first address (E01 on the sticker) is used for wan (ether1 in OEM).
- The next address is used for lan2.
- The last address (E06 on the sticker) is used for sfp.

[Initial port work, shared dtsi]
Signed-off-by: Vince Grassia <vincenzo.grassia@zionark.com>
[SFP support and GPIO identification]
Signed-off-by: Luka Logar <luka.logar@iname.com>
[Misc. fixes and submission]
Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
[rebase, drop uart3 from state_default on 750gr3, minor commit
title/message facelift]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>