powerpc/pseries: Distinguish between a os-partition and non-os partition

Introduce os_partition member in nvram_os_partition structure to identify
if the partition is an os partition or not. This will be useful to handle
non-os partitions of-config and common.

Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Reviewed-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>