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 receiving the MSIs from 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.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>