mips: bmips: enable the SPI flash on the Comtrend AR-5387un

It's a Macronix (mx25l12805d) 16 MB SPI flash.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>