mmc: sdhci: add a voltage switch callback function

This patch adds a callback function to do
controller-specific actions when switching voltages.
It is a preparation and will be used by Fujitsu
SDHCI controller f_sdh30 driver.

Signed-off-by: Vincent Yang <Vincent.Yang@tw.fujitsu.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>