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: 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>