PCI: cadence-ep: Introduce cdns_pcie_ep_disable() helper for cleanup

Introduce the helper function cdns_pcie_ep_disable() which will undo the
configuration performed by cdns_pcie_ep_setup(). Also, export it for use
by the existing callers of cdns_pcie_ep_setup(), thereby allowing them
to cleanup on their exit path.

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/20250417124408.2752248-4-s-vadapalli@ti.com