jlink: Add EMUCOM support

EMUCOM is a way to communicate with a J-Link device via so called
channels. A channel can either be read or written in a single
operation.

Beside the reserved channels for SEGGER, there are channels available to
implement vendor and/or device specific functionalities. For example,
EMUCOM is used on many starter and development kits from Silicon Labs to
access power measurements and various other information and settings.

Change-Id: I6094109c043b34aed4a40ceabe71f30ff896bf1d
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3794


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>