Regular rng peripheral, with one additional bit : clock error detection
apparently available on l4 chips). Curiously, Clock error detection is
_disabled_ when bit is set, but bit is cleared by default, so peripheral
/ clock error detection behaves like all other chips..
NB: RNG need proper rcc_ccicr_rngsel bits set to work, no clock is set by
default. Note also that on that chip fRNGCLK must be higher than fHCLK/32
