juntek-jds6600: implement device support, tested with Joy-IT JDS6600

juntek-jds6600: implement device support, tested with Joy-IT JDS6600

Implement support for the essential signal generator part of the device,
tested with a 60MHz Joy-IT JDS6600 model: Two channels, enable/disable,
waveform, frequency, amplitude, offset, duty cycle selection, phase
between channels.

This version does not support external signals and frequency measurement
nor counting. Upload/download of arbitrary waveforms could work from the
serial communication's perspective, but sigrok lacks infrastructure/API
for waveform exchange or communication of blobs.

Tested with sigrok-cli and getters/setters. Quickly tested with SmuView.