mmc: sdhci-sprd: Add pin control support for voltage switch

For Spreadtrum SD card voltage switching, besides regulator setting,
it also need switch related pin's state to output corresponding voltage.

This patch adds pin control operation to support voltage switch.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>