dt-bindings: rng: add MediaTek MT7622 Hardware Random Generator bindings

Document the bindings used by MediaTek MT7622 SoC hardware random number
generator.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>