mmc: mmci: Add callbacks for to manage signal voltage switch

A variant may need to define some actions before and after a voltage
switch.

This patch adds 2 callbacks to manage signal voltage switch in the struct
mmci_host_ops. ->pre_sig_volt_switch() allows to prepare a signal voltage
switch before sending the SD_SWITCH_VOLTAGE command (CMD11).
->post_sig_volt_switch callback allows specific actions to be executed,
after the I/O signal voltage level has been changed.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200128090636.13689-8-ludovic.barre@st.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>