dt-bindings: PCI: brcm,iproc-pcie: Fix 'msi' child node schema

dt-bindings: PCI: brcm,iproc-pcie: Fix 'msi' child node schema

The 'msi' child node schema is missing constraints on additional properties.
It turns out it is incomplete and properties for it are documented in the
parent node by mistake. Move the reference to msi-controller.yaml and
the custom properties to the 'msi' node. Adding 'unevaluatedProperties'
ensures all the properties in the 'msi' node are documented.

With the schema corrected, a minimal interrupt controller node is needed
to properly decode the interrupt properties since the example has
multiple interrupt parents.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Fixes: 905b986d099c ("dt-bindings: pci: Convert iProc PCIe to YAML")
Link: https://lore.kernel.org/r/20230926155613.33904-3-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>