ramips: add support for YunCore FAP640

ramips: add support for YunCore FAP640

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
- Wi-Fi:
    - MT7915DN + MT7905DAN: 2.4/5 GHz
- Ethernet: 5x1GiE via MT7530, 1xWAN with POE and 4xLAN
- UART: J4 (115200 baud)
    - Pinout: [3V3] (TXD) (RXD) (GND)
- Power: 802.11af/at PoE;

- Bootloader: U-Boot
- Buttons:
    - Reset
- LEDs:
    - Status - RGB controlled by
      - GPIO 14 LOW - green color
      - GPIO 15 LOW- red color
      - GPIO 16 LOW - blue color
    - WAN - gren color, controlled by switch GPIO 12 LOW
    - LAN1 - gren color, controlled by switch GPIO 9 HIGH
    - LAN2 - gren color, controlled by switch GPIO 6 LOW
    - LAN3 - gren color, controlled by switch GPIO 3 LOW
    - LAN4 - gren color, controlled by switch GPIO 0 LOW

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
router will not boot.

MAC addresses with 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>