usb: dwc3: of-simple: Add compatible for Spreadtrum SC9860 platform

Add compatible string to use this generic glue layer to support
Spreadtrum SC9860 platform's dwc3 controller.

Signed-off-by: Baolin Wang <baolin.wang@spreadtrum.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>