ARM: ux500: Set correct MMCI regulator voltages in the ux5x0 Device Tree

Correct the voltage specified by the mmci regulator node in Device
Tree. Despite the MMC subsystem insisting on v3.3, we actually only
offer v2.9, and not v2.6 which must have actually been a typo.

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