sequential/prophet5.cpp: routed remaining "master sum" signals.

* Implemented pitch modulation.
* Routed master frequency and pulse width CVs for oscillators A and B.
* Simplified routing of master CV.
* Collapsed some MIXER + {other} stages into the {other} stages.
* Collapsed some VA_CONST + MIXER stages into VA_SCALE_OFFSET stages.
* Reordered section in `prophet5_voice_device` stream pipeline to: osc A, osc B, filter, amplifier.

Author: m1macrophage

src/devices/sound/va_ops.h

@@ -37,7 +37,8 @@ class va_const_device : public device_t, public device_sound_interface
 
 
 // Scales and offsets the input stream. Useful for emulating certain op-amp and
-// resistor network circuits.
+// resistor network circuits. A common use is as a MIXER with an offset or
+// constant input.
 class va_scale_offset_device :  public device_t, public device_sound_interface
 {
 public: