usb: dwc2: pci: Add device mode to the dwc2-pci driver

The pci driver now registers a platform driver, like in dwc3, and lets
its probe function do all the initialization. This allows it to
account for changes to the platform driver that were not added to the
pci driver. Also future changes to the probe function don't have to be
duplicated. This also has the effect of adding device and DRD mode to
the pci driver. Tested on the Synopsys HAPS PCIe platform.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>