dt-bindings: mailbox: fsl,mu: Add missing power-domains

Add quite common property - power-domains - to fix dtbs_check warnings
like:

  arch/arm64/boot/dts/freescale/imx8qxp-mek.dt.yaml:
    mailbox@5d280000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'

Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201002161837.5784-1-krzk@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/mailbox/fsl,mu.yaml

@@ -72,6 +72,9 @@ properties:
     description: boolean, if present, means it is for side B MU.
     type: boolean
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg