powerpc/mm: Only define MAX_PHYSMEM_BITS in SPARSEMEM configurations

[Rui Salvaterra]

Hi, Ben

> Presumably you have CONFIG_PPC_BOOK3S_64 enabled and CONFIG_SPARSEMEM disabled?
> Was this configuration actually usable?

I just noticed these build warnings too. FWIW, I've been using
CONFIG_FLATMEM on my Power Mac G5 for about three years, so I guess
the answer is "yes" (for this specific machine, at least).