ar71xx: add support for Rockeetech AP531B0

Specification:
- SoC: Qualcomm Atheros QCA9531 (650/400/216 MHz (CPU/DDR/AHB))
- RAM: 64 MiB DDR2
- Storage: 16 MiB of flash (optional 16 MiB second flash)
- Wireless: built into QCA9531 (Honey Bee), PHY modes b/g/n
- Ethernet: 1x100M (port0) 1x100M (port 4)
- PCI-E: x1
- SIMcard slot: x1
- USB 2.0 port: x1 (optional)
- Ext GPIO pin: x8
- UART for serial console: x1
- External watchdog: x1 (optional)
- Power supply: 9-48V (DC jack/passive PoE)

Installation through tftp:
- Copy <Firmware Image Name> into tftp directory
- Connect board via ttl port and Ethernet port
- Boot the AP531B0
- When "Autobooting in 1 seconds" appears press Enter
- Set serverip and ipaddr appropriate
  (e.g. setenv serverip 192.168.88.10;setenv ipaddr 192.168.88.1)
- Enter "tftp 0x8050000 <Firmware Image Name>"
- Enter "erase 0x9f050000 +$filesize"
- Enter "cp.b 0x80500000 0x9f050000 $filesize"
- Enter "reset"

Signed-off-by: Shuanglin Liu <roboidler@gmail.com>