ath79: add support for TP-Link Archer C6 v2

ath79: add support for TP-Link Archer C6 v2

This patch adds support for TP-Link Archer C6 v2 (EU)

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

Flash instructions:

Upload
openwrt-ath79-generic-tplink_archer-c6-v2-squashfs-factory.bin
via the router Web interface.

Flash instruction using tftp recovery:

1. Connect the computer to one of the LAN ports of the router
2. Set the computer IP to 192.168.0.66
3. Start a tftp server with the OpenWrt factory image in the
   tftp root directory renamed to ArcherC6v2_tp_recovery.bin.
4. Connect power cable to router, press and hold the reset
   button and turn the router on
5. Keep the reset button pressed until the WPS LED lights up
6. Wait ~150 seconds to complete flashing

According to the GPL source the non-EU variant has different
GPIOs assigned to some of the LEDs and buttons. The flash
layout might be different as well. The wikidevi entry for
Archer A6/C6 assumes they are identical.

Signed-off-by: Georgi Vlaev <georgi.vlaev@gmail.com>