PCI: qcom: Handle mixed PERST#/PHY DT configuration

The driver currently supports two PERST# and PHY DT configurations. In one
case, PHY and PERST# are described in the RC node. In the other case, they
are described in the RP node.

A mixed setup is not supported. One common example is PHY on the RP node
while PERST# remains on the RC node. In that case the driver goes through
the RP parse path, does not find PERST# on RP, and does not report an error
because PERST# is optional. Probe can then succeed silently while PERST# is
left uncontrolled, and PCIe endpoints fail to work later. This silent
probe success makes debugging difficult.

Handle this mixed case in the RP parse path by checking whether PERST# is
present on RC and, if so, using the RC PERST# GPIO for RP ports while
keeping RP parsing for PHY. Emit a warning to indicate mixed DT content so
it can be fixed.

This keeps mixed systems functional and makes the configuration issue
visible instead of failing later at endpoint bring-up.

Suggested-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Qiang Yu <qiang.yu@oss.qualcomm.com>
[mani: folded the fix: https://lore.kernel.org/linux-pci/20260526-fix_perst_gpio_handling-v1-1-9170507bb4e9@oss.qualcomm.com]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://patch.msgid.link/20260508-mix_perst_phy_dts-v1-1-9eff6ee9b51a@oss.qualcomm.com

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