ramips: add support for Kroks Rt-Pot mXw DS RSIM router

ramips: add support for Kroks Rt-Pot mXw DS RSIM router

Aka "Kroks KNdRt31R19".
Ported from v19.07.8 of OpenWrt fork:
see https://github.com/kroks-free/openwrt
for support of other models.

Device specs:
- CPU: MediaTek MT7628AN
- Flash: 16MB SPI NOR
- RAM: 64MB
- Bootloader: U-Boot
- Ethernet: 1x 10/100 Mbps
- 2.4 GHz: b/g/n SoC
- mPCIe: 1x (usually equipped with an LTE modem by vendor)
- Buttons: reset
- LEDs: 1x Modem, 1x Injector, 1x Wi-Fi, 1x Status

Flashing:
- sysupgrade image via stock firmware WEB interface.
- U-Boot launches a WEB server if Reset button is held during power up.
Server IP: 192.168.1.1

SIM card switching:
The device supports up to 4 SIM cards - 2 locally on board and 2 on
remote SIM-injector.
By default, 1-st local SIM is active.
To switch to e.g. 1-st remote SIM:
echo 0 > /sys/class/gpio/modem1power/value
echo 0 > /sys/class/gpio/modem1sim1/value
echo 1 > /sys/class/gpio/modem1rsim1/value
echo 1 > /sys/class/gpio/modem1power/value

MAC addresses as verified by OEM firmware:
vendor   OpenWrt   source
LAN      eth0      factory 0x4 (label)
2g       wlan0     label

Signed-off-by: Kroks <dev@kroks.ru>
[butirsky@gmail.com: port to master; drop dts-v1]
Signed-off-by: Andrey Butirsky <butirsky@gmail.com>