arm: dts: stm32mp157c-odyssey-som: set the SDMMC2 VQMMC supply

Set the SDMMC2 VQMMC supply to vdd (buck3). Note that in the
corresponding Linux kernel device tree, the supply is set to v3v3
(buck4), which is wrong.

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>