ar71xx: add support for Samsung WAM250

ar71xx: add support for Samsung WAM250

Samsung WAM250 is a dual-band (selectable, not simultaneous) wireless
hub, dedicated for Samsung Shape Wireless Audio System. The device is
based on Atheros AR9344. FCC ID: A3LWAM250.

Specification:

- 560/450/225 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 2x 10/100 Mbps Ethernet
- 2T2R 2.4/5 GHz (AR9344), with ext. PA (SE2598L, SE5003L) and LNA
- 1x USB 2.0
- 4x LED (all are driven by GPIO)
- 2x button (reset, wps/speaker add)
- DC jack for main power input (14 V)
- UART header on PCB (J4, RX: 3, TX: 5)

Flash instruction:

This device uses dual-image (switched between upgrades) with a common
jffs2 config partition. Fortunately, there is a way to disable this mode
so that more flash space can be used by OpenWrt image.

You can easily access this device over telnet, using root/root
credentials (the same also work for serial console access).

1. Make sure that your device uses second (bootpart=2) image using
   command: "fw_printenv bootpart".
2. If your device uses first image (bootpart=1), perform upgrade to the
   latest vendor firmware (after the update, device should boot from
   second partition) using web gui (default login: admin/1234567890).
3. Rename "sysupgrade" image to "firmware.bin", download it (you can use
   wget, tftp or ftpget) to "/tmp" and issue below commands:

   mtd_debug erase /dev/mtd3 0 $(wc -c /tmp/firmware.bin | awk -F' ' '{print $1}')
   mtd_debug write /dev/mtd3 0 $(wc -c /tmp/firmware.bin)
   fw_setenv bootpart
   fw_setenv bootcmd "bootm 0x9f070000"
   reboot

Revert to vendor firmware instruction:

1. Download vendor firmware to "/tmp" device and issue below commands:

   fw_setenv bootpart 1
   sysupgrade -n -F SS_BHUB_v2.2.05.bin

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