PCI: qcom: Implement .reset_root_port() and use for link down

The PCIe link can go down under circumstances such as the device firmware
crash, link instability, etc. When that happens, the Root Port needs to be
reset to make it operational again. Currently, the driver is not handling
the link down event, so users have to restart the machine to make PCIe link
operational again. Fix it by detecting the link down event and resetting
the Root Port.

Since the Qcom PCIe controllers report the link down event through the
'global' IRQ, enable the link down event by setting PARF_INT_ALL_LINK_DOWN
in the PARF_INT_ALL_MASK register.

In the case of the event, iterate through the available Root Ports and call
pci_host_handle_link_down() API with Root Port 'pci_dev' to let the PCI
core handle the link down condition. Since Qcom PCIe controllers only
support one Root Port per controller instance, the API will be called only
once. But the looping is necessary as there is no PCI API available to
fetch the Root Port instance without the child 'pci_dev'.

The API will internally call the 'pci_host_bridge::reset_root_port()'
callback to reset the Root Port in a platform-specific way. Implement the
callback to reset the Root Port by first resetting the PCIe core, followed
by reinitializing the resources and then finally starting the link again.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
[bhelgaas: subject]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Reviewed-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Link: https://patch.msgid.link/20260729-pci-port-reset-v9-4-53570b92064d@oss.qualcomm.com

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