mmc: stm32: sdmmc2: add support for st, pin-ckinsdmmc_ckin

This patch adds "st,pin-ckin" support to activate sdmmc_ckin feature.
When using an external driver (a voltage switch transceiver),
it's advised to select SDMMC_CKIN feedback clock input to sample
the received data.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>