m68knommu: Add missing initialization of max_pfn and {min,max}_low_pfn

If max_pfn is not initialized, the block layer may use wrong DMA masks.

Replace open-coded shifts by PFN_DOWN(), and drop the "0 on coldfire"
comment, as it is not even true on all Coldfires, let alone all
m68knommu platforms.

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