rdtech-um: Add initial support for the RDTech UMxx series

This changeset adds support for the RDTech UMxx series of USB power
meters. The driver has been tested with the RDTech UM24C, but should
support the UM24C, UM25C, and the UM34C.

Currently, the driver reports the following channels:
  * V: VBus voltage
  * I: VBus current
  * D+: D+ voltage
  * D-: D- voltage
  * T: Device temperature
  * 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 a UM24C.

Missing features:

  * There is currently no support for configuring threshold-based
    recording from sigrok, but this can be done on the device itself.

  * Fast charging mode currently not logged.

Usage example:

sigrok-cli -d rdtech-um:conn=bt/rfcomm/MAC --scan
sigrok-cli -d rdtech-um:conn=/dev/rfcomm0 --scan

Known issues:

  * When using sigrok's Bluetooth transport implementation, the device
    is disconnected between probing and sampling. Some devices (e.g.,
    the UM24C), dislikes this and can't be reconnected reliably for
    sampling. This is not an issue when setting up a rfcomm device
    manually and using it as a serial port.

Kudos to Sven Slootweg for documenting most of the protocol.

Signed-off-by: Andreas Sandberg <redacted>