PCI: cadence: Add support to build pcie-cadence library as a kernel module

Currently, the Cadence PCIe controller driver can be built as a built-in
module only. Since PCIe functionality is not a necessity for booting, add
support to build the Cadence PCIe driver as a loadable module as well.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
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/20250417124408.2752248-2-s-vadapalli@ti.com