powerpc/pseries: Add PFO support to the VIO bus

Add support for the Platform Facilities Option (PFO) to the VIO bus.
These devices have a separate root node in OpenFirmware which
requires additional parsing to map into the existing VIO device
structure fields. This adds the interface for PFO device drivers to
make synchronous hypervisor calls.

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>