Documentation: devicetree: add Freescale SCC bindings

Add documentation for the Freescale Security Controller (SCC)
found on i.MX25 SoCs.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>