dt-bindings: usb: msm-phy: Add extcon binding for SSUSB QMP PHY

Add optional extcon property for SSUSB QMP PHYs. This allows
notification of various connection statuses such as the number
of DisplayPort lanes being used.

Change-Id: I26927e8849d6040422597b1b8c18270d9aab0515
Signed-off-by: Jack Pham <jackp@codeaurora.org>