dt-bindings: rng: introduce new compatible for STM32MP13x

Introduce st,stm32mp13-rng compatible and add st,rng-lock-conf.

If st,rng-lock-conf is set, the RNG configuration in RNG_CR, RNG_HTCR
and RNG_NSCR will be locked. It is supported starting from the RNG
version present in the STM32MP13

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