ar71xx: add support for YunCore SR3200 and XD3200

ar71xx: add support for YunCore SR3200 and XD3200

YunCore SR3200 is a dual-band AC1200 router, based on Qualcomm/Atheros
QCA9563+QCA9882+QCA8337N.

YunCore XD3200 (FCC ID: 2ADUG-XD3200) is a dual-band AC1200 ceiling mount
AP with PoE support, based on Qualcomm/Atheros QCA9563+QCA9882+QCA8334.

Common specification:

- 775/650/258 MHz (CPU/DDR/AHB)
- 128 MB or RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 2T2R 2.4 GHz, with ext. PA (SKY65174-21), up to 30 dBm
- 2T2R 5 GHz, with ext. PA (SKY85405-11) and LNA (SKY85601-11), up to 30 dBm

SR3200 specification:

- 5x 10/100/1000 Mbps Ethernet
- 6x ext. RP-SMA antennas (actually, only 4 are connected with radio chips)
- 3x LED (+ 5x LED in RJ45 sockets), 1x button
- UART header on PCB

XD3200 specification:

- 2x 10/100/1000 Mbps Ethernet, with 802.3at PoE support (WAN port)
- 4x internal antennas
- 3 sets of LEDs on external PCB (+ 2x LED near RJ45 sockets), 1x button
- UART and JTAG (custom 6-pin, 2 mm pitch) headers on PCB

LED for 5 GHz WLAN is currently not supported on both devices as it is
connected directly to the QCA9882 radio chip.

Flash instruction under vendor firmware, using telnet/SSH:

1. If your firmware does not have root password, go to point 5
2. Connect PC with 192.168.1.x address to LAN or WAN port
3. Power up device, enter failsafe mode with button (no LED indicator!)
4. Change root password and reboot (mount_root, passwd ..., reboot -f)
5. Upload lede-ar71xx-...-sysupgrade.bin to /tmp using SCP
6. Connect PC with 192.168.188.x address to LAN port, SSH to 192.168.188.253
7. Invoke:
- cd /tmp
- fw_setenv bootcmd "bootm 0x9fe80000 || bootm 0x9f050000"
- mtd -e firmware -r write lede-ar71xx-...-sysupgrade.bin firmware

Flash instruction under U-Boot, using UART:

1. tftp 0x80060000 lede-ar71xx-...-sysupgrade.bin
2. erase 0x9f050000 +$filesize
3. cp.b $fileaddr 0x9f050000 $filesize
4. setenv bootcmd "bootm 0x9fe80000 || bootm 0x9f050000"
5. saveenv && reset

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