rdtech-tc: Add initial support for the RDTech TC66C

This changeset adds support for the RDTech TC66C USB power meter.

Currently, the driver reports the following channels:
  * V: VBus voltage
  * I: VBus current
  * D+: D+ voltage
  * D-: D- voltage
  * E: Energy consumed in threshold-based recording mode.

The number of significant digits shown for each channel has been set
to match the number of digits shown on the device.

Usage example:

sigrok-cli -d rdtech-tc:conn=/dev/ttyACM0 --scan

Known issues:

  * BLE support is currently unimplemented. This uses a different
    command set, but the same poll data format.

Kudos to Ben V. Brown for reverse engineering some of the protocol and
documenting the encryption key used for poll data.

Signed-off-by: Andreas Sandberg <redacted>