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

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

+Device-Tree bindings for Mediatek random number generator
+found in Mediatek SoC family
+
+Required properties:
+- compatible	    : Should be "mediatek,mt7623-rng"
+- clocks	    : list of clock specifiers, corresponding to
+		      entries in clock-names property;
+- clock-names	    : Should contain "rng" entries;
+- reg 		    : Specifies base physical address and size of the registers
+
+Example:
+
+rng: rng@1020f000 {
+	compatible = "mediatek,mt7623-rng";
+	reg = <0 0x1020f000 0 0x1000>;
+	clocks = <&infracfg CLK_INFRA_TRNG>;
+	clock-names = "rng";
+};