Add SN76489 (or another PSG) chip to support true square voices

[Wohlstand]

This simple PSG chip that also was presented on Sega Megadrive game console also was used in game music as YM2612 processes sine-wave only voices and also having of SN76489 gives extra three voices and one noise channel!

SN76489 chips can be used to make existing voices sound better.

[Wohlstand]

As OPNA chip support with embedded PSG upcoming, let's put this or another PSG emulator to use with OPN2 out of OPNA.

[freq-mod]

MAME has a good SN76489A emulation core that's not ridiculously outdated - https://github.com/mamedev/mame/blob/master/src/devices/sound/sn76496.cpp so it would be useful as an OPN2 companion.

[jrmoserbaltimore]

Doesn't the OPNA come with the YMF2149 in one bank?