ar71xx: add support for Wallys DR342

ar71xx: add support for Wallys DR342

Wallys DR342 is a 5 GHz, 2T2R AP/CPE board based on Atheros AR9342.

Short specification:

- 560/450/225 MHz (CPU/DDR/AHB)
- 1x Gbps Ethernet (AR8035) with passive PoE support (24-56 V)
- 64 MB of RAM (DDR2)
- 16 MB of FLASH
- 2T2R 5 GHz with external FEM (SKY85728-11), up to 30 dBm
- 2x MMCX connectors
- miniPCIe connector with PCIe and USB 2.0 buses
- optional miniSIM slot
- 7x LED, 1x button
- UART, (E)JTAG and LED headers
- 1x DC jack for main power (12-56 V)

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

1. tftp 0x82000000 lede-ar71xx-generic-dr342-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>