ARM: ux500: Correct Snowball's external SD/MMC slot's bus width

Snowball's external SD/MMC slot is only capable of 4 bit data
transfer, not 8 bits as previously thought. This is a simple
fixup moving to the correct settings.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>