hwrng: ks-sa - add hw_random driver

Keystone Security Accelerator module has a hardware random generator
sub-module. This commit adds the driver for this sub-module.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
[t-kristo@ti.com: dropped one unnecessary dev_err message]
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>