Three subarches support hugepages:
  - fsl book3e
  - book3s/64
  - 8xx

This patch splits asm/hugetlb.h to reduce the #ifdef mess.
Signed-off-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>