MASTECH MS6514 support

libsigrok now supports the MASTECH MS6514 digital thermometer.

This is a device with two thermocouple inputs and USB connectivity (SiLabs CP2102 based). It supports K,J,T,E,R,S,N thermocouple types.

A teardown and the protocol description are available on the device's wiki page, if you're interested.

The driver was contributed by Dave Buechi, thanks a lot!