Support enabling / disabling the vqmmc power supply if it was provided
by the firmware.

Provide the .start_signal_voltage_switch callback to change the voltage
of the external vqmmc power supply.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20230131210229.68129-2-paul@crapouillou.netSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>