dt-bindings: consolidate RNG bindings

We have RNG bindings in hwrng/ and rng/. Consolidate them all under rng/.
Signed-off-by: NRob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>