dt-bindings: PCI: qcom,pcie-sa8775p: Document 'link_down' reset

Each PCIe controller on SA8775P includes a 'link_down' reset line in
hardware. This patch documents the reset in the device tree binding.

The 'link_down' reset is used to forcefully bring down the PCIe link
layer, which is useful in scenarios such as link recovery after errors,
power management transitions, and hotplug events. Including this reset
line improves robustness and provides finer control over PCIe controller
behavior.

As the 'link_down' reset was omitted in the initial submission, it is now
being documented. While this reset is not required for most of the block's
basic functionality, and device trees lacking it will continue to function
correctly in most cases, it is necessary to ensure maximum robustness when
shutting down or recovering the PCIe core. Therefore, its inclusion is
justified despite the minor ABI change.

Signed-off-by: Ziyue Zhang <ziyue.zhang@oss.qualcomm.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://patch.msgid.link/20250718081718.390790-3-ziyue.zhang@oss.qualcomm.com