dt-bindings: PCI: qcom,pcie-sa8255p: Document ECAM compliant PCIe root complex

dt-bindings: PCI: qcom,pcie-sa8255p: Document ECAM compliant PCIe root complex

Document the required configuration to enable the PCIe Root Complex on
SA8255p, which is managed by firmware using power-domain based handling
and configured as ECAM compliant.

Signed-off-by: Mayank Rana <mayank.rana@oss.qualcomm.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: add "ECAM" in reg description]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250616224259.3549811-4-mayank.rana@oss.qualcomm.com