adi_v5_swd: Read RDBUFF once after a sequence of AP reads

Increases performance by a factor of two for long reads.

Change-Id: I81a7a83835058560c6a53a43c3cc991100f01766
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1954
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

src/target/arm_adi_v5.h

@@ -187,6 +187,12 @@ struct adiv5_dap {
 	/* information about current pending SWjDP-AHBAP transaction */
 	uint8_t  ack;
 
+	/**
+	 * Holds the pointer to the destination word for the last queued read,
+	 * for use with posted AP read sequence optimization.
+	 */
+	uint32_t *last_read;
+
 	/**
 	 * Configures how many extra tck clocks are added after starting a
 	 * MEM-AP access before we try to read its status (and/or result).