usb: dt-bindings: Add qcom,default-mode-none to msm-ssusb bindings

Setting this property prevents USB to start in host or device mode
so that it goes to low power mode.

Change-Id: Icefee47a0a1a068a3c18f3a05593c883335a16b0
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>