usb: pd: Add vendor defined message handling

Add APIs to send and receive vendor defined messages (VDM) over
USB PD. A handler for a standard or vendor ID (SVID) can register
callbacks to be notified of reception of VDM messages. One
use case is for another kernel driver, such as DisplayPort, to be
able to be notified when an Alternate Mode adapter is connected
to the Type-C port in order to enter modal operation. SVID
handlers should maintain their own state and timer resources
in order to comply with the Power Delivery Specification.

Change-Id: Ibe26e6deeca587f21f8121f6f32cf7cc0a5c3e23
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>