ar71xx: add support for Wallys DR531

Wallys DR531 is based on Qualcomm Atheros QCA9531 v2.
Short specification:

- 550/400/200 MHz (CPU/DDR/AHB)
- 2x 10/100 Mbps Ethernet
- 64 MB of RAM (DDR2)
- 8 MB of FLASH
- 2T2R 2.4 GHz with external PA (SE2576L), up to 30 dBm
- 2x MMCX connectors
- mini-PCIe connector with PCIe/USB buses and SIM slot
- 7x LED, 1x button, 1x optional buzzer
- UART, (E)JTAG and LED headers

Default configuration:

- WAN on eth1 (RJ45 near DC jack)
- LAN on eth0 (RJ45 near button)
- S4 LED set to be status LED
- all LEDs configurable form user space
- button configured for reset

Flash instruction (do it under U-Boot, using UART):

1. tftp 0x80060000 lede-ar71xx-generic-dr531-squashfs-sysupgrade.bin
2. erase 0x9f050000 +$filesize
3. cp.b $fileaddr 0x9f050000 $filesize
4. setenv bootcmd "bootm 0x9f050000"
5. saveenv && reset

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