ar71xx: add support for YunCore AP90Q

YunCore AP90Q is an outdoor CPE/AP based on Qualcomm/Atheros QCA9531 v2.
Short specification:

- 650/600/216 MHz (CPU/DDR/AHB)
- 2x 10/100 Mbps Ethernet, passive PoE support
- 64/128 MB of RAM (DDR2)
- 16 MB of FLASH
- 2T2R 2.4 GHz with external PA, up to 29 dBm
- 2x internal 5 dBi omni antennas
- 4x LED, 1x button
- UART (JP1) header on PCB

Flash instruction under U-Boot, using UART:

1. tftp 0x80060000 lede-ar71xx-generic-ap90q-squashfs-sysupgrade
2. erase 0x9f050000 +$filesize
3. cp.b $fileaddr 0x9f050000 $filesize
4. setenv bootcmd "bootm 0x9f050000"
5. saveenv && reset

Flash instruction under vendor fimrware, using telnet/SSH:

1. Connect PC with 192.168.1.x address to WAN port
2. Power up device, enter failsafe mode with button (no LED indicator!)
3. Change root password and reboot (mount_root, passwd ..., reboot -f)
4. Upload lede-ar71xx-generic-ap90q-squashfs-sysupgrade.bin to /tmp using SCP
5. Connect PC with 192.168.188.x address to LAN port, SSH to 192.168.188.253
6. Invoke:
- cd /tmp
- fw_setenv bootcmd "bootm 0x9f050000"
- mtd erase firmware
- mtd -r write lede-ar71xx-generic-ap90q-squashfs-sysupgrade.bin firmware

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