PCI: qcom-ep: Add support for firmware-managed PCIe Endpoint

PCI: qcom-ep: Add support for firmware-managed PCIe Endpoint

Some Qualcomm platforms use firmware to manage PCIe resources such as
clocks, resets, and PHY through the SCMI interface. In these cases,
the Linux driver should not perform resource enable or disable
operations directly.

So introduce a `firmware_managed` flag in 'struct qcom_pcie_ep_cfg', and
set it to true for SA8255p SoC. When this flag is set, the driver will skip
the resource handling and rely on runtime PM APIs to let the firmware
handle the resources with the help of power domain.

Signed-off-by: Mrinmay Sarkar <mrinmay.sarkar@oss.qualcomm.com>
[mani: reworded description and tiny code cleanup]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20260106-firmware_managed_ep-v5-2-1933432127ec@oss.qualcomm.com