servo_v4: Added dut-control for power role swap

Added dut-control and driver method in order query the current power
role and to set it to the desired role.

BUG=b:37219390
BRANCH=none
TEST=Issued the following commands and verified that the role changed
as expected:
dut-control servo_v4_role:snk
dut-control servo_v4_role:src
Verifed that role is reported correctly in both cases.

Change-Id: I4ca2610256d9687f3847e74c48f3ee40bcc47bb7
Signed-off-by: Scott Collyer <scollyer@google.com>
Reviewed-on: https://chromium-review.googlesource.com/474209
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