usb: dwc3: of-simple: Add compatible string for Intel Keem Bay platform

Add compatible string to use this generic glue layer to support
Intel Keem Bay platform's dwc3 controller.

Signed-off-by: Wan Ahmad Zainie <wan.ahmad.zainie.wan.mohamad@intel.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>