powerpc/pseries: Enable CMO feature during platform setup

For Cooperative Memory Overcommitment (CMO), set the FW_FEATURE_CMO
flag in powerpc_firmware_features from the rtas ibm,get-system-parameters
table prior to calling iommu_init_early_pSeries.

With this, any CMO specific functionality can be controlled by checking:
 firmware_has_feature(FW_FEATURE_CMO)

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>