dt-bindings: PCI: qcom,pcie-sm8550: Document 'global' interrupt

dt-bindings: PCI: qcom,pcie-sm8550: Document 'global' interrupt

Qcom PCIe RC controllers are capable of generating 'global' SPI interrupt
to the host CPU. This interrupt can be used by the device driver to handle
PCIe link specific events such as Link up and Link down, which give the
driver a chance to start bus enumeration on its own when link is up and
initiate link training if link goes to a bad state. The PCIe driver can
still work without this interrupt but it will provide a nice user
experience when device gets plugged and removed.

Document the interrupt as optional for SM8550 and SM8650 platforms.

Link: https://lore.kernel.org/r/20241126-topic-sm8x50-pcie-global-irq-v1-1-4049cfccd073@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>