Add some PC-88 presets, with external soundcard(s)

Added AY clock (1.9968MHz) for PC-88
reference:
http://mydocuments.g2.xrea.com/html/p8/soundinfo.html,
https://www.dtmstation.com/archives/52016817.html

src/engine/platform/ay.cpp

@@ -830,6 +830,9 @@ void DivPlatformAY8910::setFlags(const DivConfig& flags) {
       case 14:
         chipClock=1536000;
         break;
+      case 15:
+        chipClock=38400*13*4; // 31948800/16
+        break;
       default:
         chipClock=COLOR_NTSC/2.0;
         break;