arm: dts: stm32mp157c-odyssey-som: enable SDMMC2 in SPL

Enable the SDMMC2 device tree node, as well as the pins it uses, in
U-Boot SPL.

Signed-off-by: Grzegorz Szymaszek <gszymaszek@short.pl>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>