hwrng: stm32 - restrain RNG noise source clock

For NIST certification the noise source sampling may need to be
restrained.

This change implements an algorithm that gets the rate of the RNG
clock and apply the correct value in CLKDIV field in RNG_CR register
to force the RNG clock rate to be "max_clock_rate" maximum.

As it is platform-specific, implement it as a compat data.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>