powerpc: Fix MSI support on U4 bridge PCIe slot

On machines using the Apple U4 bridge (AKA IBM CPC945) PCIe interface such
as the latest generation G5 machines x16 slot or the x16 slot of the
PowerStation, MSIs are currently broken (and will oops when enabling).

This fixes the oops and implements proper support for those. Instead of
using the PCIe <-> HT bridge conversion, on such slots we need to use
a bunch of magic registers in the bridge as the MSI target, encoding
the interrupt number in the low bits of the address itself

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>