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>

tcl/board/icnova_imx53_sodimm.cfg

@@ -17,6 +17,10 @@ echo "i.MX53 SO-Dimm board lodaded."
 
 adapter_khz 3000
 
+# Slow speed to be sure it will work
+jtag_rclk 1000
+$_TARGETNAME configure -event "reset-start" { jtag_rclk 1000 }
+
 $_TARGETNAME configure -event "reset-assert" {
 	echo "Reseting ...."
 	#cortex_a dbginit