dt-bindings: PCI: layerscape-pci: Add deprecated property 'num-viewport'

Copy the 'num-viewport' property from snps,dw-pcie-common.yaml to
fsl,layerscape-pcie.yaml to address the following warning:

  /arch/arm64/boot/dts/freescale/fsl-ls1012a-frwy.dtb: pcie@3400000: Unevaluated properties are not allowed ('num-viewport' was unexpected)

This is necessary due to historical reasons where fsl,layerscape-pcie.yaml
does not directly reference snps,dw-pcie-common.yaml.

Link: https://lore.kernel.org/linux-pci/20240823185855.776904-1-Frank.Li@nxp.com
Signed-off-by: Frank Li <Frank.Li@nxp.com>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/pci/fsl,layerscape-pcie.yaml

@@ -69,6 +69,14 @@ properties:
     minItems: 1
     maxItems: 2
 
+  num-viewport:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    deprecated: true
+    description:
+      Number of outbound view ports configured in hardware. It's the same as
+      the number of outbound AT windows.
+    maximum: 256
+
 required:
   - compatible
   - reg