ARM: new DPM interface

ARM: new DPM interface

First version of interface for sharing code between ARMv6 and ARMv7a
debug modules ... now the architecture includes debug support.  (Not
the same as for the trimmed-down v7m or v6m though!)  This is a first
version of an interface that will let the ARM11 and Cortex-A8 support
share code, features, and bugfixes.  Based on existing code from both
of those cores.

The ARM v7-AR architecture specification calls this commonality the
"Debug Programmer's Model (DPM)", which seemed to be an appropriate
acronym -- a TLA even! -- for use in our code.  Made it so.  :)

The initial scope of this just supports register access, and is geared
towards supporting top level "struct arm" mechanisms.  Later, things
like breakpoint and watchpoint support should be included.

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