dt-bindings: rng: Document BCM7278 RNG200 compatible

BCM7278 includes a RGN200 hardware random number generator, document the
compatible string for that version of the IP.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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

 HWRNG support for the iproc-rng200 driver
 
 Required properties:
-- compatible : "brcm,iproc-rng200"
+- compatible : Must be one of:
+	       "brcm,bcm7278-rng200"
+	       "brcm,iproc-rng200"
 - reg : base address and size of control register block
 
 Example: