rng: stm32: add RNG clock frequency restraint

In order to ensure a good RNG quality and compatibility with
certified RNG configuration, add RNG clock frequency restraint.

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