ARM: ux500: Setup correct settling time for the MMCI regulator

The GPIO controlled MMCI regulator used on the ux5x0 boards takes
100us to settle. There's already a binding to provide such
information. Let's make use of it.

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