usb: dwc3: add a platform device alias

We can have three modules here: dwc3.ko, dwc3-omap.ko and dwc3-pci.ko.
The later have already ids-aliases for probing and is fine. The omap
module has alias for DT but lacks alias for the "native"
platform_device. Maybe we should get rid of it and stick to the DT name?
Both glue modules create a new device for which the dwc3.ko module is
responsible and that one lacks the platform alias.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>