crypto: caam - set RDB bit in security configuration register

This change is required for post SEC-5.0 devices which have RNG4.
Setting RDB in security configuration register allows CAAM to use the
"Random Data Buffer" to be filled by a single request. The Random Data
Buffer is large enough for ten packets to get their IVs from a single
request. If the Random Data Buffer is not enabled, then each IV causes a
separate request, and RNG4 hardware cannot keep up resulting in lower
IPSEC throughput if random IVs are used.

Signed-off-by: Vakul Garg <vakul@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>