usb: dwc2: remove dwc2_platform.ko

As dwc2 pci module is now exporting dwc2 platform device, include
platform.o in dwc2-y and remove USB_DWC2_PLATFORM configuration
option. Driver will be built as two modules, dwc2.ko and dwc2_pci.ko.
dwc2.ko is the new platform driver.

Remove all EXPORT_SYMBOL_GPL as they are not needed any more.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>