AM335x: allow simultaneous debugging of A8 and M3 cores

AM335x: allow simultaneous debugging of A8 and M3 cores

This patch fixes the tap order so that it matches the actual jtag
chain when all taps are enabled. It also introduces a variable
DEFAULT_TAPS that can be set outside of this script, e.g. on the
command line, to specify which taps are to be enabled on init.
Lastly, a new debug target "am335x.m3" is added so that the Wakeup-M3
can be selected for debugging.

Change-Id: Iccf177fda8d5e3737b1b2bb8fd1eaa7d3262ed9f
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3013
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>