Documentation/bindings: Document the SafeXel cryptographic engine driver

The Inside Secure Safexcel cryptographic engine is found on some Marvell
SoCs (7k/8k). Document the bindings used by its driver.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>