jtag/drivers/jlink: implement register command to fix SWD

Some J-Link fw versions require registration to be performed before
SWD operation is possible. It doesn't harm anyway, vendor's utilities
do it unconditionally.

Thanks go to Segger for providing the necessary information.

Change-Id: Iabd76c743eca86e2c817a97cb93c969fec3f7ac6
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2331
Tested-by: jenkins