usb: pd: Read sink capabilities from device tree

Add a device tree property to allow specifying the supported
set of sink capabilities. If not specified, fall back to a
single capability of 5V @ 3A.

Change-Id: I394061d9f3070099e35b651ce4b63a7993343bf1
Signed-off-by: Jack Pham <jackp@codeaurora.org>