ARM: dts: msm: Set default mode for usb as none for sdx55

This particular platform doesn't support detection of cable
connect or disconnect, so set the default mode as none for USB
so that the USB subsystem stays in low power mode. And if user
wants to change the mode, they need to use the device attribute
"mode" to change into the required peripheral or host mode.

Change-Id: I693d23ee45c54d9bd09aaf1e5b1dbcdd84be4d7c
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>