ramips: add support for Buffalo WCR-1166DS

Buffalo WCR-1166DS is a small wireless router with

 - MT7628AN + MT7612E
 - 64MiB DDR2 SDRAM
 - 16MiB SPI flash
 - 2T2R 11ac/a/b/g/n Wi-Fi
 - 2x 10/100M ethernet switch
 - 8x programmable LED
 - 3x button
 - UART pad on PCB (J2: 3.3V, GND, TX, RX)

factory image can be installed via stock web UI.

due to the "dual image" function in the bootloader, the second half of
the SPI flash ("firmware2" partition) cannot be used as a part of the
file system.

Signed-off-by: FUKAUMI Naoki <naobsd@gmail.com>