patch for am7930 audio

[Tetsuya Isaki <isaki%pastel-flower.jp@localhost>]

I have improved am7930 family audio drivers to share more code among
them: audioamd(4) on sparc, vsaudio(4) on vax, and bba(4).

The patch is: http://www.netbsd.org/~isaki/20200904-am7930.diff

vsaudio(4) and bba(4) were tested by naru@, tsutsui@.  Both worked
fine.  (thanks!)

By the way, I'm not so familiar to sparc.
audioamd(4) has many assembly code (though they look very old stuff).
Is there any (historical or traditional) reason why I should not
remove these?

Summary of this patch:
- Remove complex and useless callbacks: onopen, onclose, and
  indirect_{read,write}.  This makes audioamd(4) and vsaudio(4) almost
  the same.
- Remove all assembly code for audioamd(4).
- trigger_* method (as pseudo DMA) is more suitable for this device.

Thanks,
---
Tetsuya Isaki <isaki%pastel-flower.jp@localhost / isaki%NetBSD.org@localhost>