dt-bindings: hwrng: Add Mediatek hardware random generator bindings

Document the devicetree bindings for Mediatek random number
generator which could be found on MT7623 SoC or other similar
Mediatek SoCs.

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