Emulate Atari STE LMC1992 tone and volume hardware

[frno7]

Some SNDH files made for the Atari STE make use of the LMC1992 circuit for tone and volume control. Let PSG play emulate this piece of hardware:

The LMC1992 is a monolithic integrated circuit that providesfour stereo inputs, bass and treble tone controls, and volume, balance, and front-rear fader controls. These functions are digitally controlled through a three-wire communication interface.

Plan:

• define mixer device in lib/atari/mixer.c;
• read and write Microwire registers;
• mixer control;
• main volume control;
• left and right volume controls;
• bass control;
• treble control.

[frno7]

LMC1992 main, left and right volume controls in commit 3a98699. Interactive text mode volume control with - and + keys in commit cad18e7.