servo_v4: Added dut-control for DTS mode on/off

Servo_v4 has a console command 'dts on|off' which is used to enable or
disable DTS mode. When this mode is disabled, servo_v4 DUT port will
connect to the DUT as a regular SRC UFP. When enabled, it connects as
a DTS which triggers CCD mode to be enabled.

This CL adds a dut-control to access this EC console command. Added
both a getter and a setter method.

BUG=b:36908939
BRANCH=none
TEST=Launch servod for servo_v4. Tested the following:
dut-control servo_v4_dts_mode
dut-control servo_v4_dts_mode:on
dut-control servo_v4_dts_mode:off

Change-Id: I2bce7a70297aa93bbf7b6f4f5faa3291c40b9cfb
Signed-off-by: Scott Collyer <scollyer@google.com>
Reviewed-on: https://chromium-review.googlesource.com/470332
Commit-Ready: Scott Collyer <scollyer@chromium.org>
Tested-by: Scott Collyer <scollyer@chromium.org>
Reviewed-by: Wai-Hong Tam <waihong@google.com>
2 files changed