PCI: designware: Set up high part of MSI target address

Set up the high part of the MSI target address to allow the MSI target to
be above 4GB on 64bit and PAE systems.

[bhelgaas: changelog]
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>