dwc3: Handle optional clocks

Usually dwc3 needs a glue node that contain the SoC specific clocks/resets.
For some reason the RK3328 DTS doesn't have this glue node and the clocks
are specified in the dwc3 node directly.
The bindings says that it is allowed but doesn't specified some strict names
for them.
Add a specific case for RK3328 based on the compatible string.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D37392
Sponsored by: Beckhoff Automation GmbH & Co. KG