usb: pd: Support VCONN Swap

Support incoming VCONN Swap requests by accepting and turning
off/on VCONN.

Due to HW board limitations, if VCONN is being sourced from the
VBUS input we cannot support enabling VCONN while as a sink and
greater than 5V has been negotiated on VBUS. In that case, reject
the request. Add a device tree property that indicates whether
the board is configured for separate VCONN supply.

Change-Id: If3a9aa316ae08a80468631f3d536a1b345e21b18
Signed-off-by: Jack Pham <jackp@codeaurora.org>