PCI: imx6: Add helper function imx_pcie_add_lut_by_rid()

Add helper function imx_pcie_add_lut_by_rid(), which will be used by the
upcoming LUT configuration for MSI/IOMMU in the Endpoint mode. No
functional change.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[mani: reworded commit message and dropped tested-by tag]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250710-ep-msi-v21-1-57683fc7fb25@nxp.com