dt-bindings: PCI: dwc: improve msi handling

Allow missing "msi" interrupt, iff the node has a "msi-map" property.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20230731165723.53069-2-sebastian.reichel@collabora.com


Signed-off-by: Rob Herring <robh@kernel.org>