PCI: layerscape: Modify the MSIX to the doorbell mode

dw_pcie_ep_raise_msix_irq was never called in the exisitng driver
before, because the ls1046a platform don't support the MSIX feature
and msix_capable was always set to false.
Now that add the ls1088a platform with MSIX support, use the doorbell
method to support the MSIX feature.

Link: https://lore.kernel.org/r/20200918080024.13639-9-Zhiqiang.Hou@nxp.com


Signed-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: Andrew Murray <andrew.murray@arm.com>