rdtech-dps: introduce support for RD6006 and other Riden RD models

rdtech-dps: introduce support for RD6006 and other Riden RD models

The RD devices differ from DPS devices in their default communication
bitrate (115200), register content endianess (BE), and register set
layout (addresses for registers). In either case 16bit registers get
accessed by means of Modbus communication over a serial channel. The
interpretation of the registers' values mostly is the same as for DPS
devices once a register with the same meaning got identified. Device
identification includes a 32bit serial number which DPS devices appear
to not provide.

All other product features are the same, and map to identical sigrok
mechanisms. That's why re-using the rdtech-dps driver to add rdtech-rd
support is considered desirable. This implementation shares all code
"above" the raw register addressing and raw value interpretation. All
logical processing, configuration interface, state tracking and data
submission to the session feed, are shared among the different device
types.

Declare support for the RD6006, RD6012, and RD6018 models. Their specs
were taken from the 2020.12.2 instruction manual. The driver was tested
with an RD6006 device (firmware version 1.28). Unfortunately automatic
device detection is not possible or will not be reliable, which is why
users need to specify the respective model by picking one of the two
drivers. Within a driver variant the device identification and use of
the device are automatically dealt with.