powerpc: Fix build error for book3e

We moved the definition of shift_to_mmu_psize and mmu_psize_to_shift
out of hugetlbpage.c in patch "powerpc: New hugepage directory format".

These functions are not related to hugetlbpage and we want to use them
outside hugetlbpage.c We missed a definition for book3e when we moved
these functions. Add similar functions to mmu-book3e.h

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>