Subsystem: Add HierarchicalLocation

[rmac-sifive]

Related issue:

Type of change: other enhancement

Impact: API addition (no impact on existing code)

Development Phase: implementation

Release Notes
Adding HierarchicalLocation which config keys may use to control hierarchy of instantiation

[hcook]

Can you take the one from my PR?
https://github.com/chipsalliance/rocket-chip/pull/2327/files#diff-3d87f78ef4648ef6b47c853c4581a47bR17

and then the two objects supply string names matching their own class names?

[hcook]

Sorry, might be a bug in my PR, but here is my code proposal:

 class HierarchicalLocation(override val name: String) extends Location[LazyScope](name)    
 case object InTile extends HierarchicalLocation("InTile")                                  
 case object InSubsystem extends HierarchicalLocation("InSubsystem")                        
 case object InSystem extends HierarchicalLocation("InSystem")