Depending on the CONFIG selected, many of the MMU features are
not possible. Lets only get the possible ones in MMU_FTRS_POSSIBLE.

This allows gcc to get rid at compile time of code related to
not possible features.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>