arm: dts: stm32mp157c-odyssey-som: enable the RNG1

Enable the true random number generator. It can be used, for example, to
generate partition UUIDs when partitioning with the gpt command. The
generator is already enabled in the device trees of several other
STM32MP1‐based boards, like DKx or DHCOM.

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: Patrick Delaunay <patrick.delaunay@foss.st.com>