ramips: add support for xiaomi RA75 Range Extender

ramips: add support for xiaomi RA75 Range Extender

This device is a 'Range extender' variant of the Xiaomi 4A router.
Its identical to the 100m non-intl/chinese version as much as it can run
the same firmware, differences being form factor, LEDs, WPS button
and one 100M port only.
The stock firmware differs significantly, being 'app managed only'.

Device specification
--------------------
SoC:      MT7628DAN MIPS_24KEc@580MHz 2.4G-n 2x2
WiFi:     MT7612EN 5G-ac 80MHz 2T2R
Flash:    16MB W25Q128BV
DRAM:     64MB built-in SoC
Switch:   built-in SoC
Ethernet: 1x10/100 Mbps
USB:      None
Antennas: 2 x external, non-detachable
LEDs:     2 programmable blue/amber
Buttons:  WPS and reset (hidden)
Housing:  Range Extender / Wall wart
Serial:   115200,8n1

MAC Addresses
-------------
All 3 MACs are read from flash and identical to stock.
Label MAC is WIFI 2G

Installation
------------
No HTML UI on this device, serial console only. The serial connector
is unpopulated but standard size and clearly marked. Flash from the
U-Boot shell at boot by choosing (2) and flashing the sysupgrade file
via tftp.

Recovery/Debricking procedures of the xiaomi 4A and variants should
work, but there currently is no official source for the stock firmware.

Signed-off-by: Jo Deisenhofer <jo.deisenhofer@gmail.com>