ar71xx: Add support for Mikrotik RB SXT 2nD r3

ar71xx: Add support for Mikrotik RB SXT 2nD r3

The Mikrotik RouterBOARD SXT 2nD r3 is an outdoor WiFi AP / CPE
with a single 2.4 GHz radio and a 100 Mbps Ethernet port.
The device similar to the SXT 2nD r2, but it has SPI NOR flash instead
of NAND flash.

Hardware
--------
CPU:   Atheros AR9344 (600 MHz)
RAM:   64 MiB
FLASH: 16 MiB SPI NOR W25Q128
ETH:   1x 100 Mbps Atheros AG71xx
WiFi: 2T2R 802.11b/g/n (ath9k)
Power: Passive PoE 8-30 V

Installation instructions:

1. Boot openwrt-ar71xx-mikrotik-vmlinux-initramfs.elf using a
   DHCP+TFTP server.
2. Erase the "firmware" partition using the mtd command. This should
   no longer be required once this patch is merged.
3. Use sysupgrade to install to flash. The file
   openwrt-ar71xx-mikrotik-rb-nor-flash-16M-squashfs-sysupgrade.bin
   should be used.

Signed-off-by: Xavier Douville <github@douville.org>