m68k/mm: sun3 - Add missing initialization of max_pfn and {min,max}_low_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.

Switch from init_bootmem_node() to init_bootmem(), as there's only one
memory node on Sun-3. This will initialize min_low_pfn and max_low_pfn,
which was also not done before.

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