dt-bindings: PCI: qcom,sa8255p-pcie-ep: Document firmware managed PCIe endpoint

dt-bindings: PCI: qcom,sa8255p-pcie-ep: Document firmware managed PCIe endpoint

Document the required configuration to enable the PCIe Endpoint controller
on SA8255p which is managed by firmware using power-domain based handling.

Signed-off-by: Mrinmay Sarkar <mrinmay.sarkar@oss.qualcomm.com>
[mani: added MAINTAINERS entry]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260106-firmware_managed_ep-v5-1-1933432127ec@oss.qualcomm.com