ath79-generic: add support for TP-Link Archer C6v2

Hardware specification:
- SOC: Qualcomm QCA9563 @ 775MHz
- Flash: GigaDevice GD25Q64CSIG (8MiB)
- RAM: Zentel A3R1GE40JBF (128 MiB DDR2)
- Ethernet: Qualcomm QCA8337N: 4x 1Gbps LAN + 1x 1Gbps WAN
- Wireless:
  - 2.4GHz (bgn) QCA9563 integrated (3x3)
  - 5GHz (ac) Qualcomm QCA9886 (2x2)
- Button: 1x power, 1x reset, 1x wps
- LED: 6x LEDs: power, wlan2g, wlan5g, lan, wan, wps
- UART: There's no UART header on the board