Need annotations to drive clock constraints

[jwright6323]

Maybe this belongs in testchipip? We need to have a way to annotate ClockMux, ClockDivider etc. to generate HammerIR for clock constraints.

[edwardcwang]

We are planning to add these kinds of standard annotations / transforms in the Hammer (IR) Scala library to address this kind of issue (a standard way to annotate and generate Hammer IR from Chisel/FIRRTL).