powerpc/pseries: Enable the PFO-based RNG accelerator

This patch adds the cas bits to advertise support for the Platform
Facilities Option (PFO) based random number generator accerator.
The pseries-rng driver provides support for this hardware feature.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Signed-off-by: Kent Yoder <key@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>