jtag: tcl: Add cget -idcode

This can be used to dynamically distinguish between similar chips in tcl
code.

Change-Id: Ic4dfb61693e78616355173142dc7f9b9683a9f73
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-on: http://openocd.zylin.com/5033


Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>