dt-bindings: PCI: apple: Add missing 'power-domains' property

The 'unevaluatedProperties' schema checks is not fully working and doesn't
catch some cases where there's a $ref to another schema. A fix is pending,
but results in new warnings in examples.

The Apple PCIe host schema is missing 'power-domains' in the schema.
The example has 3 power domains. However, this is wrong too as actual
dts files have a single power domain and Sven confirmed 1 is correct.

Cc: Sven Peter <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Reviewed-by: Sven Peter <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/pci/apple,pcie.yaml

···
   iommu-map: true
   iommu-map-mask: true
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg
···
         ranges = <0x43000000 0x6 0xa0000000 0x6 0xa0000000 0x0 0x20000000>,
                  <0x02000000 0x0 0xc0000000 0x6 0xc0000000 0x0 0x40000000>;
 
-        power-domains = <&ps_apcie>, <&ps_apcie_gp>, <&ps_pcie_ref>;
+        power-domains = <&ps_apcie_gp>;
         pinctrl-0 = <&pcie_pins>;
         pinctrl-names = "default";