ARM: 7724/1: mmc: mmci: Support signal voltage switch for UHS cards

Add .start_signal_voltage_switch callback to be able to support UHS cards.
The voltage switch requires the optional vqmmc regulator to exist since
the actual voltage switch will be performed directly on it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>