powerpc/mm: Add MMU_FTR_KERNEL_RO to possible feature mask

commit a5ecdad4847897007399d7a14c9109b65ce4c9b7 upstream.

Without this we will always find the feature disabled.

Fixes: 984d7a1e ("powerpc/mm: Fixup kernel read only mapping")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>