ARM: ux500: Use the GPIO regulator framework for SDI0's 'en' and 'vsel'

To prevent lots of unnecessary call-backs into platform code, we're
now using the GPIO regulator framework to control the 'enable' (en)
and 'voltage select' (vsel) GPIO pins which in turn control the
MMCI's secondary regulator settings. This already works with Device
Tree, but when booting with ATAGs we need to register it as a
platform device.

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