target: Pull out the jtag_rtck from iMX5x files

target: Pull out the jtag_rtck from iMX5x files

Pull the jtag_rtck setting from imx51.cfg and imx53.cfg . Since
not all boards using these CPUs do support RTCK signal, move the
configuration of RTCK into board files.

Change-Id: I632c5d38e00ada8779a451cd26428fd122452001
Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Reviewed-on: http://openocd.zylin.com/1460
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>