mmc: stm32_sdmmc2: remove privdata

All the elements of privdata are static and build from device tree,
they are moved in platdata to prepare the support of ops
of_to_plat.

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