mach-ux500: Add SDI support for snowball board

With SDI support for the Snowball we can boot from the
SD card.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Robert Marklund <robert.marklund@stericsson.com>