[POWERPC] Prepare for splitting up mmu.h by MMU type

Currently asm-powerpc/mmu.h has definitions for the 64-bit hash based
MMU.  If CONFIG_PPC64 is not set, it instead includes asm-ppc/mmu.h
which contains a particularly horrible mess of #ifdefs giving the
definitions for all the various 32-bit MMUs.

It would be nice to have the low level definitions for each MMU type
neatly in their own separate files.  It would also be good to wean
arch/powerpc off dependence on the old asm-ppc/mmu.h.

This patch makes a start on such a cleanup by moving the definitions
for the 64-bit hash MMU to their own file, asm-powerpc/mmu_hash64.h.
Definitions for the other MMUs still all come from asm-ppc/mmu.h,
however each MMU type can now be one-by-one moved over to their own
file, in the process cleaning them up stripping them of cruft no
longer necessary in arch/powerpc.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>