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 <[e-mail]>
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

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

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