dt-bindings: usb: Add gsi-disable-io-coherency to driver bindings

This allows to disable io-coherency in usb gsi for msm-ssusb driver.

Change-Id: I63278351f65ce9b117d5e85d783ccb9873a2a0ae
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>