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 <sven@svenpeter.dev>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Link: https://lore.kernel.org/r/20220531215815.2408477-1-robh@kernel.org