powerpc/eeh: Add missing procfs entry for PowerNV

The procfs entry for global statistics has been missed on PowerNV
platform and the patch is going to add that.

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