powerpc64: support superpages on pmap_mincore

Now that superpages for HPT MMU has landed, finish implementation of
pmap_mincore by adding support for superpages.

Submitted by: Fernando Eckhardt Valle <fernando.valle@eldorado.org.br>
Reviewed by: bdragon, luporl
MFC after: 1 week
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D29230