Add TileId parameter, generalizing GroundTestId

This usually shouldn't be used in Tiles that are meant to be P&R'd once
and multiply instantiated, as their RTL would no longer be homogeneous.
However, it is useful for conditionalizing RTL generation for
heterogeneous tiles.

src/main/scala/rocket/tile.scala

@@ -12,6 +12,7 @@ import cde.{Parameters, Field}
 case object BuildRoCC extends Field[Seq[RoccParameters]]
 case object NCachedTileLinkPorts extends Field[Int]
 case object NUncachedTileLinkPorts extends Field[Int]
+case object TileId extends Field[Int]
 
 case class RoccParameters(
   opcodes: OpcodeSet,