ARM: ux500: Disable the MMCI gpio-regulator by default

Not all supported boards will require a MMCI gpio-regulator,
therefore it's a good idea to only enable the node when and if
it is required. Let's disable it by default.

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