pci_ep: Add the init function

Some EP deivces need to initialize before RC scan it, e.g. NXP
layerscape platform, so add the init function in pci_ep uclass.

Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>