powerpc/mm: Add missing pmd accessors

This patch add documentation and missing accessors.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --cc arch/powerpc/include/asm/pgtable-ppc64-4k.h
Simple merge

diff --cc arch/powerpc/include/asm/pgtable-ppc64-64k.h
Simple merge

diff --cc arch/powerpc/include/asm/pgtable-ppc64.h
Simple merge

diff --cc arch/powerpc/mm/hugetlbpage.c
Simple merge

diff --cc arch/powerpc/mm/pgtable_64.c
Simple merge