Implement support for the (optional) MSI controller of Synopsys Designware

Implement support for the (optional) MSI controller of Synopsys Designware
PCIe host bridge.  This MSI controller is quite retarded since it maps all
MSIs to a single hardware interrupt.  So it doesn't really offer any
benefit over using classic INTx interrupts.  Unfortunately we need to use
it on Amlogic SoCs since the PCIe device interrupt doesn't seem to work
correctly when configured as a level triggered interrupt and the workaround
of configuring it as an edge triggered interrupt causes problems when using
multiple disks connected to ahci(4) on the ODROID-HC4.

ok patrick@