cubieboard: Enable the SD controller and mux SD pins

This step needs to be done before calling any MMC functionality.

Change-Id: I88763072c8a541ddba794e79fb55e82eb2f187a9
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/4745
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@gmail.com>