m68k/mm: motorola - Add missing initialization of max_pfn

If max_pfn is not initialized, the various /proc/kpage* files are empty,
and selftests/vm/mlock2-tests will fail. max_pfn is also used by the
block layer to calculate DMA masks.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@uclinux.org>