Add ops to support USB PD VDM (Vendor Defined Message) from the port
driver. This enables the port driver to interface with alternate mode
drivers and communicate with connected peripherals.

The initial support just contains an implementation of the Enter
Mode command.

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Prashant Malani <pmalani@chromium.org>
[pmalani: Fixed trivial conflict in Makefile]
Reviewed-by: Benson Leung <bleung@chromium.org>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20221228004648.793339-9-pmalani@chromium.org