powerpc/mm/nohash: Move definition of PGALLOC_GFP to fix build errors

In some obscure Book3E configs (randconfig) we can end up missing a
definition for PGALLOC_GFP in pgtable_64.c.

Fix it by moving the definition to asm/pgalloc.h.

Fixes: de3b8761 ("powerpc/mm/book(e)(3s)/64: Add page table accounting")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>