sound/cem3394.cpp: improved CEM3394 and adapted sente6vb to the changes.

[m1macrophage]

Added and improved CEM3394 functionality relevant to the sixtrak while working on audio for that driver.

Sending as a separate PR to keep the scope manageable, given there are also changes to sente6vb.

Incidental improvements to sente6vb:

• Unlocked some sounds. An obvious one is the post-tuning chirp for each voice during boot.
• Centered audio around 0.

sound/cem3394 changes:

• Made VCO output range [-1, 1], instead of [0, 1].
• Added pulse-width dependent DC level of pulse waveform, as described in the datasheet.
• Waveform selection logic now works for voltages outside the bounds provided in the datasheet.
  • Both the sixtrak and sente6vb will some times use voltages outside those ranges.
• Added AC coupling between VCF output and VCA input.
• Forcing parameter computation in device_start().
• Added ability to stream voltage inputs. The sixtrak needs that for the VCA gain and VCF frequency.
• New configuration interface:
  • New configure() method computes VCO and VCF zero frequencies based on the attached passive components.
  • Exposed method for enabling pulse-width limiting for drivers that need it. Now disabled by default.
• Fixed compilation for FILTER_TYPE_ESQ1.

midway/sente6vb changes:

• Using new CEM3394 configuration interface.
  • Verified that it produces almost the same VCO and VCF zero frequencies.
• Reduced CEM3394->speaker gain to compensate for increased loudness of CEM3394
  • Old range: [0, 1] nominal. New range: ~[-1, 1] x 0.5 = ~[-0.5, 0.5] nominal (can be exceeded in some cases).
• Added AC coupling to noise output.
  • Also changed the noise->CEM3394 gain from 0.5 to 1.0, to keep relative loudness the same.
  • Old range: [0, 1] x 0.5 = [0, 0.5]. New range: [-0.5, 0.5] x 1