ARM: ux500: Add a VQMMC (level-shifting) regulator DT node for Snowball

Until recently platform code took care of all MMCI level-shifting by
way of an ios_handler() call-back. Now it is the driver's responsibility
to handle. In order to so that we need to provide the VQMMC regulator
reference in Device Tree. This patch takes care of that.

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