dt-bindings: PCI: qcom: Document msi-map and msi-map-mask properties

dt-bindings: PCI: qcom: Document msi-map and msi-map-mask properties

The Qcom PCIe controller is capable of using either internal MSI controller
or the external GIC-ITS for signaling MSIs sent by endpoint devices.
Currently, the binding only documents the internal MSI implementation.

Let's document the GIC-ITS imeplementation by making use of msi-map and
msi-map-mask properties. Only one of the implementation should be used
at a time and the drivers can choose the preferred one.

Link: https://lore.kernel.org/r/20230411121442.22227-1-manivannan.sadhasivam@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>