powerpc/powernv: Support inbound error injection

For now, we only support outbound error injection. Actually, the
hardware supports injecting inbound errors as well. The patch enables
to inject inbound errors.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>