PCI: imx6: Delay link start until configfs 'start' written

According to Documentation/PCI/endpoint/pci-endpoint-cfs.rst, the Endpoint
controller (EPC) should only start the link when userspace writes '1' to
the '/sys/kernel/config/pci_ep/controllers/<EPC>/start' attribute, which
ultimately results in calling imx_pcie_start_link() via
pci_epc_start_store().

To align with the documented behavior, do not start the link automatically
when adding the EP controller.

Fixes: 75c2f26d ("PCI: imx6: Add i.MX PCIe EP mode support")
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[mani: reworded commit subject and description]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20250709033722.2924372-3-hongxing.zhu@nxp.com