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

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>