[POWERPC] Split out asm-ppc/mmu.h portions for the "classic" hash-based MMU

arch/powerpc still relies on asm-ppc/mmu.h for most 32-bit MMU types.
This is another step towards fixing this.  It takes the portions
of asm-ppc/mmu.h related to the "classic" 32-bit hash page table MMU
which are still relevant in arch/powerpc and puts them in a new
asm-powerpc/mmu-hash32.h, included when appropriate from
asm-powerpc/mmu.h.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>