powerpc/mm: Add mask of possible MMU features

Follow the example of the cpu feature code, and add a mask of possible
MMU features, MMU_FTRS_POSSIBLE.

This is used in mmu_has_feature(), which allows the possible mask to act
as a shortcut for any features that are not possible, but still allows
the feature bit itself to be defined.

We will use this in the next commit to allow MMU_FTR_RADIX checks to be
elided when MMU_FTR_RADIX is not possible.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>