pci: layerscape-ep: Add check of the PCIe controller enablement

Stop to initialize the PCIe controller if it's disabled by RCW.

Fixes: 118e58e2 ("pci: layerscape: Split the EP and RC driver")
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>