driver: tminstrument: add backends for selected instruments

Add convenient functionality for:
- Rigol Technologies DP832, 3-channel laboratory power supply
- Siglent Technologies SDM3065X-SC laboratory DMM
- RND 320-KD3305P, 3-channel laboratory power supply
- ITECH Electronics IT-M3632, power supply and regenerative DC load

Signed-off-by: Martin Sabol <martin.sabol@seznam.cz>

Author: Sabolik