rng: stm32: Implement configurable RNG clock error detection

RNG clock error detection is now enabled if the "clock-error-detect"
property is set in the device tree.

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>