Add PCIe EP mode support for ls1088a and ls2088a, there are some
difference between LS1 and LS2 platform, so refactor the code of
the EP driver.

Link: https://lore.kernel.org/r/20200918080024.13639-10-Zhiqiang.Hou@nxp.comSigned-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>