PCI: designware: Add get_msi_data() to pcie_host_ops

Add a struct pcie_host_ops .get_msi_data() method for platforms to return
their special MSI message data.

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Mohit KUMAR <mohit.kumar@st.com>