DMA support for SCSI adapter of AV Mac

[Rin Okuyama <rokuyama%rk.phys.keio.ac.jp@localhost>]

Hi,

I updated the patch for DMA support for SCSI adapter of AV Mac
(Quadra / Centris 660AV / 840AV), written by Michael Zucca in
PR port-mac68k/24883.

http://www.netbsd.org/~rin/avdma_20181002.patch

With this patch, performance of disk IO improves by nearly 10
times in comparison with PIO. Also, loss of clock interrupts
due to heavy SCSI PIO are not observed anymore as far as I
can see.

Although more works may be needed before committing, the patch
works fine without any problem on my 840AV nearly two weeks.
Any comments and suggestions are welcomed, especially from
users of 660AV.

For more details, please refer to port-mac68k/24883:

http://gnats.netbsd.org/24883

Thanks,
rin