hwrng: stm32 - support RNG configuration locking mechanism

If "st,rng-lock-conf" DT binding property is set for a stm32-rng node,
the RNG configuration will be locked until next hardware block reset
or platform reset.

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