dt-bindings: PCI: snps,dw-pcie-ep: Add tx_int{a,b,c,d} legacy IRQs

The DWC core has four interrupt signals: tx_inta, tx_intb, tx_intc, tx_intd
that are triggered when the PCIe controller (when running in Endpoint mode)
has sent an Assert_INTA Message to the upstream device.

Some DWC controllers have these interrupt in a combined interrupt signal.

Add the description of these interrupts to the device tree binding.

Link: https://lore.kernel.org/linux-pci/20240607-rockchip-pcie-ep-v1-v5-3-0a042d6b0049@kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>

Author: floatious

Documentation/devicetree/bindings/pci/snps,dw-pcie-ep.yaml

@@ -151,6 +151,15 @@ properties:
             Application-specific IRQ raised depending on the vendor-specific
             events basis.
           const: app
+        - description:
+            Interrupts triggered when the controller itself (in Endpoint mode)
+            has sent an Assert_INT{A,B,C,D}/Desassert_INT{A,B,C,D} message to
+            the upstream device.
+          pattern: "^tx_int(a|b|c|d)$"
+        - description:
+            Combined interrupt signal raised when the controller has sent an
+            Assert_INT{A,B,C,D} message. See "^tx_int(a|b|c|d)$" for details.
+          const: legacy
         - description:
             Vendor-specific IRQ names. Consider using the generic names above
             for new bindings.