Layerscape PCIe has its own MSI implementation.

Register ls_pcie_msi_host_init() to avoid using DesignWare's MSI.

[bhelgaas: add comment]
Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>