mmc: sdhci-of-arasan: Add sdhci_arasan_set_power

The power register needs to have a valid voltage set
even when the power supply is managed by  an external regulator.

Signed-off-by: Milan Stevanovic <milan.o.stevanovic@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>