powerpc: define support for 16G hugepages

The huge page size is defined for 16G pages.  If a hugepagesz of 16G is
specified at boot-time then it becomes the huge page size instead of the
default 16M.

The change in pgtable-64K.h is to the macro pte_iterate_hashed_subpages to
make the increment to va (the 1 being shifted) be a long so that it is not
shifted to 0.  Otherwise it would create an infinite loop when the shift
value is for a 16G page (when base page size is 64K).

Signed-off-by: Jon Tollefson <kniht@linux.vnet.ibm.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>