mmc: mmci_sdmmc: Implement signal voltage callbacks

To prepare the voltage switch procedure, the VSWITCHEN bit must be set
before sending the CMD11. To confirm completion of voltage switch, the
VSWEND flag must be checked.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200128090636.13689-9-ludovic.barre@st.com


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