PCI: designware: Make get_msi_addr() return phys_addr_t, not u32

Make get_msi_addr() return phys_addr_t, not u32.  This allows the MSI
target address to be above 4GB for 64bit or PAE systems.

No functional change for the current 32bit platform users as phys_addr_t
maps to u32 for them.

[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>