PCI: imx6: Add LUT configuration for MSI/IOMMU in Endpoint mode

Add LUT entry for MSI/IOMMU in Endpoint mode by calling
imx_pcie_add_lut_by_rid() helper function. Since only one physical function
is supported in the Endpoint mode for now, '0' is passed as the Device ID.
This sets up a single LUT entry required for MSI/IOMMU.

The Endpoint function can operate without LUT configuration if neither
IOMMU nor MSI is used by the platform. This LUT configuration is used for
the EP doorbell feature by allowing the Root Complex to trigger the
doorbell on the Endpoint with the help of the Endpoint MSI controller.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[mani: reworded the comments & commit message and dropped tested-by tag]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250710-ep-msi-v21-2-57683fc7fb25@nxp.com