mmc: stm32: sdmmc2: add hardware flow control support

The hardware flow control functionality is used to avoid
FIFO underrun (TX mode) and overrun (RX mode) errors.
The behavior is to stop SDMMC_CK during data transfer and
freeze the SDMMC state machines.

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