target/arm_cti: add new 'ack' and 'channel' commands

target/arm_cti: add new 'ack' and 'channel' commands

these commands have been introduced to ease the manipulation of CTI trough
script files, these commands are:
 - $cti_name ack $event : to acknowledge a CTI event
 - $cti_name channel $channel_number $operation: to perform an operation
    on a specific channel, the possible operations are:
    gate, ungate, set, clear and pulse

Change-Id: I35463867a3c85072f3776c3aeb1e5788953ec435
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5315
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins