RegMapper: regmap(...) now takes BYTE addresses

If a device has configurable bus-width, we need a stable way of
enumerating registers. The byte offset stays unchanged.

This change also makes it possible to put an arbitrary number of RegFields
starting at some address which are then chopped up into appropriately bus-
sized registers.

src/main/scala/uncore/tilelink2/RegField.scala

@@ -85,7 +85,9 @@ case class RegField(width: Int, read: RegReadFn, write: RegWriteFn)
 
 object RegField
 {
+  // Byte address => sequence of bitfields, lowest index => lowest address
   type Map = (Int, Seq[RegField])
+
   def apply(n: Int)            : RegField = apply(n, (), ())
   def apply(n: Int, rw: UInt)  : RegField = apply(n, rw, rw)
   def r(n: Int, r: RegReadFn)  : RegField = apply(n, r, ())