Add an interface to retrieve servo board version

The method returns servo board version as a string like "servo_v2".

BUG=chromium:268534
TEST=Call the method and check the returned string.

Change-Id: Ie3b0b17facc686f62a6736c1534d280f7068ac45
Signed-off-by: Vic (Chun-Ju) Yang <victoryang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/186070
Reviewed-by: Yusuf Mohsinally <mohsinally@chromium.org>
1 file changed