dt-bindings: usb: dwc3: add reference clock period

Document the snps,ref-clock-period-ns property that describes reference
clock period when it deviates from the default set value.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/f5ea5bc3664a98a684ad4b699a1ac610e847176f.1630389452.git.baruch@tkos.co.il


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>