usb: dwc3: core: Add constants relating to power down

usb: dwc3: core: Add constants relating to power down

These constants aren't used anywhere yet but having in the header
could be useful for someone that wanted to use them in the future.

BUG=chrome-os-partner:11740
TEST=Kernel still compiles

Change-Id: I3181e7bfc34edad0a9bc0f9f80bf53f36e7b5768
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/30786
Reviewed-by: Olof Johansson <olofj@chromium.org>