dt-bindings: rng: add bindings doc for Keystone SA HWRNG driver

The Keystone SA module has a hardware random generator module.
This commit adds binding doc for the KS2 SA HWRNG driver.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>