ramips: Add support for TP-Link TL-WPA8631P v3

ramips: Add support for TP-Link TL-WPA8631P v3

AV1300 Gigabit Passthrough Powerline ac Wi-Fi Extender

Specifications
--------------
* SoC: MediaTek MT7621AT
* CPU: 880 MHz MIPS 1004KEc dual-core CPU
* RAM: 64 MiB DDR2 (Zentel A3R12E40DBF-8E)
* Flash: 8 MiB SPI NOR (GigaDevice GD25Q64CSIG)
* Ethernet: SoC built-in Switch 5x 1GbE
  * Port 0: PLC (connected through AR8035-A)
  * Port 1-3: LAN
* WLAN: 2x2 2.4GHz 300 Mbps + 2x2 5GHz 867 Mbps (MT7603EN + MT7613BEN)
* PLC: HomePlug AV2 (Qualcomm QCA7500)
* PLC Flash: 2MiB SPI NOR (GigaDevice GD25Q16CSIG)
* Buttons: Reset, LED, Pair, Wi-Fi
* LEDs: Power (green), PLC (green/amber), LAN (green), 2.4G (green),
  5G (green)
* UART: J1 (57600 baud)
  * Pinout: (3V3) (GND) (RX) (TX)
  * Visually identify GND from connection to PCB ground plane

Installation
------------
Installation is possible from the OEM web interface. Make sure to install
the latest OEM firmware first, so that the PLC firmware is at the latest
version. However, please first check the OpenWRT Wiki page for
confirmation that your OEM firmware version is supported.

Signed-off-by: Joe Mullally <jwmullally@gmail.com>