usb: pd: Add definitions for the Enter_USB message

usb: pd: Add definitions for the Enter_USB message

Version 2.0 of the USB Power Delivery Specification R3.0
defines a new message called Enter_USB, which is made with
USB4 in mind. Enter_USB message is in practice the same as
the Enter Mode command (used when entering alternate modes)
that just needs to be used when entering USB4 mode.

The message does also support entering USB 2.0 or USB 3.2
mode instead of USB4 mode, but it is only required with
USB4. I.e. with USB2 and USB3 Enter_USB message is optional.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20191230142611.24921-9-heikki.krogerus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>