Define MIFMasterTagBits as # bits a master can *use* in tag

junctions/src/main/scala/memserdes.scala

@@ -8,6 +8,7 @@ import cde.{Parameters, Field}
 case object MIFAddrBits extends Field[Int]
 case object MIFDataBits extends Field[Int]
 case object MIFTagBits extends Field[Int]
+case object MIFMasterTagBits extends Field[Int]
 case object MIFDataBeats extends Field[Int]
 
 trait HasMIFParameters {