PCI: j721e: Add support to build as a loadable module

The 'pci-j721e.c' driver is the application/glue/wrapper driver for the
Cadence PCIe Controllers on TI SoCs. Implement support for building it as a
loadable module.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://patch.msgid.link/20250417124408.2752248-5-s-vadapalli@ti.com