arm: dts: stm32mp157c-odyssey-som: enable the SDMMC2 eMMC HS DDR mode

Enable the SDMMC2 eMMC high-speed DDR mode as it is done in the
corresponding Linux kernel device tree.

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>