powerpc/mpc85xx: Add MSI support for Freescale PowerPC SoCs

Freescale SoCs use a set of IRQs at the high end of the OpenPIC IRQ
list, not counted in the NIRQs of the Feature reporting register. Some
SoCs include a MSI inbound window in the PCIe controller configuration
registers as well, but some don't. Currently, this only handles the
SoCs *with* the MSI window.

There are 256 MSIs per MSI bank (32 per MSI IRQ, 8 IRQs per MSI bank).
The P5020 has 3 banks, yielding up to 768 MSIs; older SoCs have only one
bank.