ARM: start abstracting ADIv5 transports (JTAG/SWD)

To support both JTAG and SWD, ADIv5 needs DAP operations which are
transport-neutral, instead being of JTAG-specific.  This patch:

 - Defines such a transport-neutral interface, abstracting access
   to DP and AP registers through a conceptual queue of operations.

 - Builds the first implementation of such a transport with the existing
   JTAG-specific code.

In contrast to the current JTAG-only interface, the interface adds
support for two previously-missing (and unused) DAP operations:

 - aborting the current AP transaction (untested);
 - reading the IDCODE register (tested) ... required for SWD init.

The choice of transports may be fixed at the chip, board, or JTAG/SWD
adapter level.  Or if all the relevant hardware supports both transport
options, the choice may be made at runtime, This patch provides basic
infrastructure to support whichever choice is made.

The current "JTAG-only" transport choice policy will necessarily continue
for now, until SWD support becomes available in OpenOCD.  Later patches
start phasing out JTAG-specific calls in favor of transport-neutral calls.

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

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