ath79: add support for YunCore A770

ath79: add support for YunCore A770

YunCore A770 is a ceiling AC750 AP with 2 Fast Ethernet ports, PoE
(802.3at) support, based on QCA9531 + QCA9887.

Specification:

- 650/597/216 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of flash (SPI NOR)
- 2x 10/100 Mbps Ethernet (PoE 802.3at support in WAN port)
- 2T2R 2.4 GHz (QCA9531), with ext. PA and LNA
- 1T1R 5 GHz (QCA9887), with ext. FEM (SKY85728-11)
- 2x regular LED, 1x RGB LED (all driven by GPIO)
- 1x button (reset)
- DC jack for main power input (12 V)
- UART header on PCB

Flash instruction:

1. First, gain root access to the device, following below steps:

- Login into web gui (default password/IP: admin/192.168.188.253).
- Go to 'Advanced' -> 'Management' -> 'System' and download backup of
  configuration (bakfile.bin).
- Open the file as tar.gz archive, edit/update 'shadow' file and change
  hash of root password to something known.
- Repack the archive, rename it back to 'bakfile.bin' and use to
  restore configuration of the device.
- After that, device will reboot and can be accessed over SSH.

2. Then, install OpenWrt:

- Login over SSH and issue command:
  fw_setenv bootcmd "bootm 0x9f050000 || bootm 0x9fe80000"
- Upload 'sysupgrade' image and install it (only if previous command
  succeeded) with command: 'sysupgrade -n -F openwrt-...'.

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