mmc: sdhci-xenon: add set_power callback

Xenon sdh controller requests proper SD bus voltage select
bits programmed even with vmmc power supply. Any reserved
value(100b-000b) programmed in this field will lead to controller
ignore SD bus power bit and keep its value at zero.
Add set_power callback to handle this.

Signed-off-by: Zhoujie Wu <zjwu@marvell.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Fixes: 3a3748db ("mmc: sdhci-xenon: Add Marvell Xenon SDHC core functionality")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>