mmc: stm32_sdmmc2: introduce of_to_plat ops

Add the uclass ops of_to_plat to parse the device tree properties
to respect the expected sequence by the driver model.

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