srd: Split USB decoder in two PDs (which stack).

srd: Split USB decoder in two PDs (which stack).

The 'usb_signalling' decoder just decodes symbols from D+/D- levels,
handles bit stuffing and outputs the symbols and (potential) packets.

The 'usb_protocol' decoder takes that output and tried to parse USB
packets from it (SOF, SETUP, IN, OUT, DATA0, and so on).
This is the base decoder on top of which various others will stack
later on.

The two new PDs are work in progress, so we still keep the old 'usb' PD
around for a little while, until the two new ones are fully working and
well-tested.