subsystem: update InterruptBusWrapper crossing code

[hcook]

This updates InterruptBusWrapper in the following backwards-compatible ways:

• It becomes a SimpleLazyModule with HasClockCrossings
• But the wrapper layer is later inlined by FIRRTL
• Parameterized clock crossing helper methods are exposed
• But the legacy connection functions are preserved
• No more use of deprecated IntXing

The beneficial effect beyond just modernization of APIs and more parameterizable crossing types is that the synchronization registers made by each crossing type will now be instantiated inside the InterruptBusWrapper hierarchically, each taking its clock and reset as their own. While in many cases the wrapper's clock and reset are the same as the subsystem clock and reset in which the from methods are presumed to be called (e.g. by the peripherals' attachTo methods), in certain cases these methods do get called inside some other scope. Thus this change should ensure that the synchronizing registers are always put in the destination clock domain, i.e. the one containing the InterruptBusWrapper and the PLIC, and not the hierarchy where the peripheral itself is attached.

Type of change: feature request

Impact: API addition (no impact on existing code)

Development Phase: implementation

Release Notes
Synchronizers

[rmac-sifive]

I was initially thinking it'd be nice to keep the optionality to instantiate synchronizers in the source domain, but on second thought I can't think of scenarios in which a designer would ever want to do that