arm_debug: optimize DP and AP reads over JTAG

arm_debug: optimize DP and AP reads over JTAG

On JTAG, all reads are pipelined. If you read a register, the result is not
delivered inside the request that issued the read, it is delivered in the
following request. The current code therefore issues a scan of the RDBUFF
register after each read. This adds a superfluous transaction after each
read.

This patch follows a strategy similar to what SWD already implements.
It also leverages that all JTAG reads are pipelined, i.e. the result
will be clocked out in the next JTAG data phase, no matter if it's
READ or WRITE. Therefore it's never necessary to explicitly read RDBUFF
other than for the very last READ before a dap_run().

Change-Id: Ie40b1fef3203f0cdcb503f40dcbd2a68b0f9776c
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3167
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>