powerpc/mm: define TOP_ZONE as a constant

The zone that contains the top of memory will be either ZONE_NORMAL
or ZONE_HIGHMEM depending on the kernel config. There are two functions
that require this information and both of them use an #ifdef to set
a local variable (top_zone). This is a little silly so lets just make it
a constant.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Cc: linux-mm@kvack.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>