drivers: entropy: stm32: limit driver to SoC supporting it

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>