usb: dwc3: Look up reference clock DT phandle in both controller DT nodes

usb: dwc3: Look up reference clock DT phandle in both controller DT nodes

There are currently four disparate placement possibilities of DWC3
reference clock phandle in SoC DTs:
- in top level glue node, with generic subnode without clock (ZynqMP)
- in top level generic node, with no subnode (i.MX8MQ)
- in generic subnode, with other clock in top level node (i.MX8MP)
- in both top level node and generic subnode (Rockchip)

Cover all the possibilities here by looking into both nodes, start
with the top level node as that seems to be used in majority of DTs
to reference the clock.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>