Only 3 subarches support huge pages. So when it is either 2 of them,
it is not the third one.

And mmu_has_feature() is known by all subarches so IS_ENABLED() can
be used instead of #ifdef
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>