dslogic: Add support for voltage threshold

dslogic: Add support for voltage threshold

The DSLogic provides two FPGA images: one for 3.3V and the other for 5V logic.
The DSLogic Pro allows to set an arbitrary voltage threshold via USB command.

This commit adds support for the DSLogic to load the FPGA image according to
an user-selectable voltage threshold.

For the DSLogic Pro, one of two fixed voltage thresholds are set, depending on
the user-selected value.

Tested with DSLogic and DSLogic Pro.

Signed-off-by: Diego Asanza <redacted>
Tested-by: Andrew Bradford <redacted>