The different PCIe controller in one board may be have different
capability of MSI or MSIX, so change the way of getting the MSI
capability, make it more flexible.

Link: https://lore.kernel.org/r/20200918080024.13639-8-Zhiqiang.Hou@nxp.comSigned-off-by: NXiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: NHou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NRob Herring <robh@kernel.org>