Initial support for bcm2838 RNG

[Robert Crowston]

I have made a first cut at supporting the Broadcom 2838 hardware random number generator, as found on the Raspberry Pi 4.

Diff: https://github.com/freebsd/freebsd/compare/master...RobCrowston:pi4-hwrng

This extends the existing bcm2835_rng.c driver to function on the Pi4. Unfortunately I do not have a Raspberry Pi 3 board to confirm it still works there, but on my Pi4, it generates (apparently) random numbers.

Some rough edges, but it works.

-- Rob Crowston.