ath79: add support for TP-Link TL-WDR6500 v2

ath79: add support for TP-Link TL-WDR6500 v2

This ports the TP-Link TL-WDR6500 v2 from ar71xx to ath79.

Specifications:

  SoC: QCA9561
  CPU: 750 MHz
  Flash: 8 MiB (Winbond W25Q64FVSIG)
  RAM: 128 MiB
  WiFi 2.4 GHz: QCA956X 3x3 MIMO 802.11b/g/n
  WiFi 5 GHz: QCA9882-BR4A 2x2 MIMO 802.11a/n/ac
  Ethernet: 4x LAN and 1x WAN (all 100M)
  USB: 1x Header

Flashing instructions:

  As it appears, the device does not support flashing via GUI or
  TFTP, only serial is possible.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Xiaobing Luo <luoxiaobing0926@gmail.com>