Chisel compile warning: Chisel3 package import

[ingallsj]

Related issue: follow-up to #2357
Type of change: paying off technical debt
Impact: no functional change
Development Phase: implementation

Release Notes
fix these Chisel compile warnings:

[warn] /rocket-chip/src/main/scala/amba/apb/Monitor.scala:17:67: type Reset in package core is deprecated (since 3.2): Use the version in chisel3._
[warn]   def legalize(bundle: APBBundle, edge: APBEdgeParameters, reset: Reset): Unit
[warn]                                                                   ^
[warn] /rocket-chip/src/main/scala/amba/axi4/Monitor.scala:17:69: type Reset in package core is deprecated (since 3.2): Use the version in chisel3._
[warn]   def legalize(bundle: AXI4Bundle, edge: AXI4EdgeParameters, reset: Reset): Unit
[warn]                                                                     ^