hwrng: brcm63xx - document device tree bindings

Documents device tree bindings for random number generator present on Broadcom
BCM6368 SoC.
Signed-off-by: NÁlvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/brcm,bcm6368.txt

+BCM6368 Random number generator
+
+Required properties:
+
+- compatible : should be "brcm,bcm6368-rng"
+- reg : Specifies base physical address and size of the registers
+- clocks : phandle to clock-controller plus clock-specifier pair
+- clock-names : "ipsec" as a clock name
+
+Example:
+	random: rng@10004180 {
+		compatible = "brcm,bcm6368-rng";
+		reg = <0x10004180 0x14>;
+
+		clocks = <&periph_clk 18>;
+		clock-names = "ipsec";
+	};