dt/bindings: Add bindings for hisilicon random number generator

Document the devicetree bindings for the random number generator found
on Hisilicon Hip04 and Hip05 soc.
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: NRob Herring <rob@kernel.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/hisi-rng.txt

+Hisilicon Random Number Generator
+
+Required properties:
+- compatible : Should be "hisilicon,hip04-rng" or "hisilicon,hip05-rng"
+- reg : Offset and length of the register set of this block
+
+Example:
+
+rng@d1010000 {
+	compatible = "hisilicon,hip05-rng";
+	reg = <0xd1010000 0x100>;
+};