ARM: ux500: Specify which IOS regulator to use for MMCI

In an effort to move platform specific GPIO controlled regulators
out from platform code we've created a new mechanism to specify
them from within the MMCI driver using the supply name 'vmmc-ios'.
For that to happen when booting device tree, we need to supply it
in the MMCI (SDI) node.

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