powerpc: function to allocate gigantic hugepages

The 16G page locations have been saved during early boot in an array.  The
alloc_bootmem_huge_page() function adds a page from here to the
huge_boot_pages list.

Acked-by: Adam Litke <agl@us.ibm.com>
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>