feat(st-crypto): manage RNG customization with device tree

Implement an optional way of RNG configuration via the device tree which
will override default hard-coded one.
It concerns both NIST and health test configurations.

Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@foss.st.com>
Change-Id: Ie4525392b889784d6d4feb655b74b96a4c849257