usb: xhci-dwc3: Add support for clocks/resets

Some platforms, like the Allwinner H6, do not have a separate glue layer
around the dwc3. Instead, they rely on the clocks/resets/phys referenced
from the dwc3 DT node itself. Add support for enabling the clocks/resets
referenced from the dwc3 DT node.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>