Silicon/Broadcom/Bcm283x: Add Bcm2838 RNG driver

This adds the Bcm2838 Random Number Generator Driver, which
is also the type of RNG that the Bmc2711-based Raspberry Pi 4
uses.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>