dt-bindings: PCI: Remove obsolete .txt docs

The content in these files has been moved to the schemas in dtschema.
pci.txt is covered by pci-bus-common.yaml and pci-host-bridge.yaml.
pci-iommu.txt is covered by pci-iommu.yaml. pci-msi.txt is covered in
msi-map property in pci-host-bridge.yaml.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Cc: Frank Li <Frank.li@nxp.com>
Link: https://patch.msgid.link/20250404221559.552201-1-robh@kernel.org

Author: robherring

Documentation/devicetree/bindings/pci/nvidia,tegra194-pcie-ep.yaml

@@ -74,7 +74,7 @@ properties:
 
   reset-gpios:
     description: Must contain a phandle to a GPIO controller followed by GPIO
-      that is being used as PERST input signal. Please refer to pci.txt.
+      that is being used as PERST input signal.
 
   phys:
     minItems: 1