hwrng: stm32 - set default random quality

Add a default quality to hw_random device to be
automatically set as new default entropy. Setting
random quality will decrease the crng init time by
switching to this hardware random source.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>