ath79: add support for Wallys DR531

ath79: add support for Wallys DR531

Wallys DR531 is based on Qualcomm Atheros QCA9531 v2. Support for this
device was first introduced in e767980eb8 (ar71xx target).

Specifications:

- Qualcomm/Atheros QCA9531 v2
- 550/400/200 MHz (CPU/DDR/AHB)
- 2x 10/100 Mbps Ethernet
- 64 MB of RAM (DDR2)
- 8 MB of flash (SPI NOR)
- 2T2R 2.4 GHz Wi-Fi, with external PA (SE2576L), up to 30 dBm
- 2x MMCX connectors (optional IPEX/U.FL)
- mini PCIe connector (PCIe/USB buses and mini SIM slot)
- 7x LED, 1x button, 1x optional buzzer
- UART, JTAG and LED headers on PCB

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

  tftpb 0x80060000 openwrt-ath79-...-dr531-squashfs-sysupgrade.bin
  erase 0x9f050000 +$filesize
  cp.b $fileaddr 0x9f050000 $filesize
  setenv bootcmd "bootm 0x9f050000"
  saveenv && reset

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