psgpam(4) audio driver using YM2149 on LUNA added

See the following source-changes log for details:

https://mail-index.netbsd.org/source-changes/2022/06/10/msg139155.html

> Log Message:
> luna68k: add psgpam(4) audio for LUNA, using YM2149 PSG/SSG sound chip.
> 
> "PSG PAM" is a audio driver using "Pulse Amplitude Modulation" with the
> YM2149 PSG/SSG chip connected to the I/O processor HD647180X on LUNA.
> The driver plays up to 30 kHz PCM audio by manipulating PSG channel volumes
> at extremely high speed with 6.144 MHz HD647180X, a Z80 compatible sub CPU.
>  https://www.youtube.com/watch?v=fR7PqaLpijQ
>  https://www.youtube.com/watch?v=kCDZCgkwgBA
> 
> Compatibilities with the existing XPIOCDOWNLD ioctl(2) to download and
> exec user programs (PSG sound driver player ported from PC-6001 etc.)
> on the HD647180X is still provided.
>  https://twitter.com/tsutsuii/status/1533247362965315584
> 
> This psgpam(4) driver for NetBSD/luna68k was written by @moveccr
> during isaki-audio2 branch developments, and it was also demonstrated
> at Open Source Conference 2018 Kyoto:
>  https://github.com/isaki68k/netbsd-src/tree/audio2
>  https://mail-index.netbsd.org/netbsd-advocacy/2018/08/07/msg000781.html
>  https://twitter.com/tsutsuii/status/1025504875625365505
> 
> Note specifications of interrupts from the HD647180X to the host LUNA
> was confirmed by Kenji Aoyama on his LUNA-88K2 running OpenBSD/luna88k.
> 
> I would like to thank all people still working on ancient LUNA machines!

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Izumi Tsutsui