dt-bindings: usb: snps,dwc3: Allow power-domains property

The Rockchip RK3399 DWC3 node has 'power-domains' property which isn't
allowed by the schema:

usb@fe900000: Unevaluated properties are not allowed ('power-domains' was unexpected)

Allow DWC3 nodes to have a power-domains entry. We could instead move
the power-domains property to the parent wrapper node, but the could be
an ABI break (Linux shouldn't care). Also, we don't want to encourage
the pattern of wrapper nodes just to define resources such as clocks,
resets, power-domains, etc. when not necessary.

Reviewed-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230124025936.3256213-1-robh@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/snps,dwc3.yaml

@@ -91,6 +91,16 @@ properties:
         - usb2-phy
         - usb3-phy
 
+  power-domains:
+    description:
+      The DWC3 has 2 power-domains. The power management unit (PMU) and
+      everything else. The PMU is typically always powered and may not have an
+      entry.
+    minItems: 1
+    items:
+      - description: Core
+      - description: Power management unit
+
   resets:
     minItems: 1