New protocol decoder: ssi32

libsigrokdecode now supports another new protocol decoder: ssi32.

This PD decodes the SSI32 protocol, which is based on SPI (hence the decoder stacks on top of the SPI PD).

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 and the example files were contributed by Oleksij Rempel, thanks a lot!