usb: pd: Add functionality to allow DP client to request DP mode

This change adds functionality to allow DP client to request to
release SS lanes for 4 lane functionality. This is allowed once
per USB type C session. Currently there is no other transition
between NO LANE <-> 2 LANE <-> 4 LANE allowed.

Change-Id: Ia6d6307d424f79ed7df2752e26e118a14c724665
Signed-off-by: Mayank Rana <mrana@codeaurora.org>