arm64: Add support for bcm2838 RNG

The hardware random number generator of the RPi4 differs slightly
from the version found on the RPi3.

This commit extends the existing bcm2835_rng driver to function on the RPi4.

Submitted by: James Mintram <me at jamesrm dot com>
Reviewed by: markm, cem, delphij
Approved by: csprng(cem, markm)
Differential Revision: https://reviews.freebsd.org/D22493

(cherry picked from commit e797dc58bd29c5bc0873fc620fc11d5332f90e7f)