dt-bindings: PCI: host-generic-pci: Allow IOMMU and MSI properties

Allow 'iommu-map', 'iommu-map-mask', and 'msi-parent' properties for
generic host. This fixes unevaluated property warnings on Arm Juno, AMD
Seattle, and FSL LS1028a.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220728175137.1172841-1-robh@kernel.org