m68k/mac: Enable PDMA for PowerBook 500 series

I can confirm that mac_scsi PDMA now works on these machines.
This increases sequential read throughput by a factor of 4.5.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>