dt-bindings: PCI: rockchip-dw-pcie: Fix description of legacy IRQ

dt-bindings: PCI: rockchip-dw-pcie: Fix description of legacy IRQ

The descriptions of the combined interrupt signals (level1) mention
all the lower interrupt signals (level2) for each combined interrupt,
regardless if the lower (level2) signal is RC or EP specific.

E.g. the description of "Combined system interrupt" includes rbar_update,
which is EP specific, and the description of "Combined message interrupt"
includes obff_idle, obff_obff, obff_cpu_active, which are all EP specific.

The only exception is the "Combined legacy interrupt", which for some
reason does not provide an exhaustive list of the lower (level2) signals.

Add the missing lower interrupt signals: tx_inta, tx_intb, tx_intc, and
tx_intd for the "Combined legacy interrupt", as per the rk3568 and rk3588
Technical Reference Manuals, such that the descriptions of the combined
interrupt signals are consistent.

Link: https://lore.kernel.org/linux-pci/20240607-rockchip-pcie-ep-v1-v5-5-0a042d6b0049@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>