ARM: ux500: Enable Device Tree support mmci for Snowball

Allow proper initialisation for MMC via the mmci driver
for the Snowball low-cost development board using DT. At
the moment we continue to use DMA setup from platform code.
Once the DMA generic DT bindings have been completed we
can then port the remainder over to DT.

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