mmc: sdhci: Export sdhci_start_signal_voltage_switch() in sdhci.c

Export sdhci_start_signal_voltage_switch() from sdhci.c.
Thus vendor sdhci driver can implement its own signal voltage
switch routine.

Signed-off-by: Hu Ziji <huziji@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>