ramips: add support for YunCore FAP690

ramips: add support for YunCore FAP690

It is an in-wall 802.11ax (Wi-Fi 6) router, based on MediaTek MT7621A.

Specifications:
- SoC: MT7621AT (880MHz, 2 Cores)
- RAM: 128 MB
- Flash: 16 MB SPI NOR
- Wi-Fi:
    - MT7915DN + MT7905DAN: 2.4/5 GHz
- Ethernet: 1x 1GiE via MT7530
- UART: J4 (115200 baud)
    - Pinout: [3V3] (TXD) (RXD) (GND)

- Bootloader: U-Boot
- Buttons:
    - SW1 - no label on the box, combined with led
- Led: Status. RGB controlled by
    - GPIO 14 - green color
    - GPIO 15 - red color
    - GPIO 16 - blue color

Installation:
OEM firmware is based on LEDE with custom UI and support standard sysupgrade
variant of firmware. However it requires "*.ubin" extension for sysupgrade file.
Always select "Factory reset" switch on upgrade to OpenWRT, otherwise
it will not boot.

MAC addresses as verified by OEM firmware:
vendor   source
LAN      factory 0x4 (label)
5g       factory 0x4 (label)
2g       label with flipped bits bit in 1-st byte and bits 5, 6, 7 in
	4-th byte

Example
label:  44:xx:xx:b7:xx:xx
lan:    44:xx:xx:b7:xx:xx
2g      46:xx:xx:c7:xx:xx
5g      44:xx:xx:b7:xx:xx

Signed-off-by: Volodymyr Puiul <volodymyr.puiul@gmail.com>