Documentation: devicetree: add Freescale RNGC binding

Add binding documentation for the Freescale RNGC found on
some i.MX2/3 SoCs.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>