[POWERPC] Split out asm-ppc/mmu.h portions for PowerPC 8xx

arch/powerpc still relies on asm-ppc/mmu.h for some 32-bit MMU types.
This patch is another step towards fixing this.  It takes the portions
of asm-ppc/mmu.h related to 8xx embedded CPUs which are still relevant
in arch/powerpc and puts them in a new asm-powerpc/mmu-8xx.h,
included when appropriate from asm-powerpc/mmu.h.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>