hwrng: iproc-rng200 - Add device tree bindings

Documents the IPROC random number generator device tree bindings
used in some Broadcom chipsets.
Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/hwrng/brcm,iproc-rng200.txt

+HWRNG support for the iproc-rng200 driver
+
+Required properties:
+- compatible : "brcm,iproc-rng200"
+- reg : base address and size of control register block
+
+Example:
+
+rng {
+        compatible = "brcm,iproc-rng200";
+        reg = <0x18032000 0x28>;
+};