usb: dt-bindings: Add dpdm-supply to msm-ssusb driver bindings

Add phandle to dpdm supply which will be used to drive dp/dm lines
in high-z state.

Change-Id: I7188bfe7b745c004ef8959d4a766e5ce52d26ac9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>