Cortex-M3: DWT cleanup/fixes

Fix the watchpoint error checks, and do them in add(), not later
in set() when it's mostly too late.  Support the full range of
watchpoint sizes (1 to 32K bytes each), and check alignments.

Minor cleanup of DWT access:  shrink lines, use "+" for address
calculations, comment a few issues.  Add debug message reporting
DWT capabilities, matching the message for FBP, and some minor
code and spec review comments.

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