servo: servo_v4: add dut_usb3_en control

dut_usb3_en enables or disables the usb3 interface
passthrough on the Parade PS8742 mux.

Note that dut_hub_usb_reset should be cycled after
in order for the USB link to be reinitialized.

BUG=chromium:718075
TEST=Check that USB3 is passed through or not.

Change-Id: I9a669a21263606b69f207f809413eb4a29c9fa3c
Reviewed-on: https://chromium-review.googlesource.com/517352
Commit-Ready: Nick Sanders <nsanders@chromium.org>
Tested-by: Nick Sanders <nsanders@chromium.org>
Reviewed-by: Alexandru M Stan <amstan@chromium.org>
3 files changed