dt-bindings: iommu: renesas,ipmmu-vmsa: Make 'power-domains' conditionally required

Fixing the compatible string typos results in an error in the example:

Documentation/devicetree/bindings/iommu/renesas,ipmmu-vmsa.example.dt.yaml:
  iommu@fe951000: 'power-domains' is a required property

Based on the dts files, a 'power-domains' property only exists on Gen 3
which can be conditioned on !renesas,ipmmu-vmsa.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: Will Deacon <will@kernel.org>
Cc: iommu@lists.linux-foundation.org
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210202205544.24812-2-robh@kernel.org