Relax master parameter of RocketCrossingParams

[jsmithsf]

Related issue:

Type of change: other enhancement

Impact: API modification

Development Phase: implementation

Release Notes

This change is meant to allow for using more generic or even specialized implementations of the master parameter of RocketCrossingKey.

@hcook I'm a little bit more concerned about my change to subsystem/Configs.scala. Right now just TileMasterPortParams is used. If another subclass gets used then WithIncoherentTiles might not do the right thing. I wasn't sure if this was the best approach.

[hcook]

Yeah, an unfortunate consequence of relaxing the type is the loss of guarantee that there exists a notional cache cork to add or remove. But the WithIncoherentTiles Config alteration is also already specific to RocketCrossingKey anyway... I think it has to be up to the SoC integrator to ensure that their equivalent of such an alteration contains the full cross product of tile crossing Fields x TilePortParams subclasses that they might be exposed to, and what to do for each combination.

A more conservative implementation would be to throw an exception upon encountering a TilePortParamsLike subclass that the alteration doesn't understand.

[jsmithsf]

@hcook does this look more acceptable?