ARM: ux500: Add node for the MMC GPIO regulator

There is a very simple mechanism to change and SDI's voltage which
consists of toggling some GPIOs. This scenario can be better handled
if we convert it to a GPIO regulator. This patch provides one which
can be read from Device Tree.

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