Expose most DWT registers via Tcl; there are a few more, but
those are mostly for profiling along with the ITM.  Having
this set available enables operations which aren't possible
with just the standard watchpoint operations.

The cycle counter may be interesting.  Turn it on after reset
by setting the LSB of the dwt_ctrl register, and it counts
CPU clocks.  You can program the comparator 0 watchpoint to
trigger on a given cycle count, rather than a data address.

Likewise, comparator 1 may be able to match data values given
address matches from one or two other comparators.  (Not all
hardware supports this capability though; try it.  That is
something the standard watchpoint methods should eventually
handle, for the single address case.)

Minor cleanup:  remove needless functional indirection for
exposing the v7m architctural registers.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>