Silicon/Broadcom/Bcm283x: Move Bcm2835 RNG defines to their own header

While the Bcm2835 and Bcm2838 RNG appear to share the same register layout,
we prefer moving the RNG specific defines into their own header in case
this assertion doesn't hold true with future RNG drivers.

Also, this should make it clearer to see which specific elements apply to
which SoC implementation.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>