PCI: host-generic: Move legacy DT binding fallback decision to caller of pci_host_common_parse_ports()

pci_host_common_parse_ports() returns -ENODEV if the bridge nodes (RP) are
not present or PERST# is only found in the Root Complex node. Then the
callers (currently just pci-imx6) assume that they need to fall back to
parsing the legacy DT binding.

But this behavior won't scale across Root Complex designs because PERST#
is not the only optional property that the callers would need to consider
for falling back to legacy binding. There could be many properties and the
API cannot incorporate all of them.

So to keep the API implementation simple, just return 0 when bridge nodes
were not found. Then it is up to the caller to use its own logic to decide
whether to fall back to legacy binding or not.

Since there is only one caller now, update the caller to skip -ENODEV check
and check for the PERST# GPIO in any of the bridge nodes and fall back to
legacy binding if not found.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
[mani: squashed imx6 patch to avoid bisectability issue, commit message]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Richard Zhu <hongxing.zhu@nxp.com>
Link: https://patch.msgid.link/20260525065443.2338629-2-sherry.sun@oss.nxp.com

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