New protocol decoder: ps2

libsigrokdecode now supports another new protocol decoder: ps2.

This PD decodes the PS/2 (low-level) protocol for keyboards, mice, and some other PS/2-attached devices.

As usual, we have a bunch of example files in the sigrok-dumps repository, and a few test-cases in the sigrok-test repo.

The decoder was contributed by Daniel Schulte, thanks a lot!