In STM32 family, RNG IP is available only on F4 and L4 series.
Besides STM32F401 does not support is neither.
Get entropy driver available on STM32 devices supporting it and
generate a compilation issue on STM32 devices that do not own
the RNG IP.
Solves #5448
Signed-Off-By: Erwan Gouriou <erwan.gouriou@linaro.org>