ath79: add support for YunCore XD4200 and A782

ath79: add support for YunCore XD4200 and A782

YunCore XD4200 ('XD4200_W6.0' marking on PCB) is Qualcomm/Atheros based
(QCA9563, QCA9886, QCA8334) dual-band, Wave-2 AC1200 ceiling AP with PoE
(802.3at) support. A782 model ('T750_V5.1' marking on PCB) is a smaller
version of the XD4200, with similar specification but lower TX power.

Specification:

- QCA9563 (775 MHz)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 2x 10/100/1000 Mbps Ethernet (QCA8334), with 802.3at PoE support (WAN)
- Wi-Fi 2.4 GHz:
  - XD4200: 2T2R (QCA9563), with ext. PA (SKY65174-21) and LNA
  - A782: 2T2R (QCA9563), with ext. FEM (SKY85329-11)
- Wi-Fi 5 GHz:
  - XD4200: 2T2R (QCA9886), with ext. FEM (SKY85728-11)
  - A782: 2T2R (QCA9886), with ext. FEM (SKY85735-11)
- LEDs:
  - XD4200: 5x (2x driven by SOC, 1x driven by AC radio, 2x Ethernet)
  - A782: 3x (1x RGB, driven by SOC and radio, 2x Ethernet)
- 1x button (reset)
- 1x UART (4-pin, 2.54 mm pitch) header on PCB
- 1x DC jack (12 V)

Flash instructions:

If your device comes with generic QSDK based firmware, you can login
over telnet (login: root, empty password, default IP: 192.168.188.253),
issue first (important!) 'fw_setenv' command and then perform regular
upgrade, using 'sysupgrade -n -F ...' (you can use 'wget' to download
image to the device, SSH server is not available):

  fw_setenv bootcmd "bootm 0x9f050000 || bootm 0x9fe80000"
  sysupgrade -n -F openwrt-...-yuncore_...-squashfs-sysupgrade.bin

In case your device runs firmware with YunCore custom GUI, you can use
U-Boot recovery mode:

1. Set a static IP 192.168.0.141/24 on PC and start TFTP server with
   'tftp' image renamed to 'upgrade.bin'
2. Power the device with reset button pressed and release it after 5-7
   seconds, recovery mode should start downloading image from server
   (unfortunately, there is no visible indication that recovery got
   enabled - in case of problems check TFTP server logs)

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>