powerpc/pseries/hwrng: PFO-based hwrng driver

Adds support for the Platform Facilities Option (PFO)-based hardware
random number generator for POWER hardware.

Signed-off-by: Michael Neuling <mikey@neuling.org>
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>