usb: dwc3: core: convert to unified device property interface

No functional affect on existing platforms, but the driver
is now ready to extract the properties also from ACPI tables
as well as from DT.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>