[POWERPC] MSI abstraction

Instead of trying to make PPC64 MSI fit in a Intel-centric MSI layer, a
simple short-term solution is to hook the pci_{en/dis}able_msi() calls
and make a machdep call.

The rest of the MSI functions are superfluous for what is needed at this
time.  Many of which can have machdep calls added as needed.

Ben and Michael Ellerman are looking into rewrite the MSI layer to be
more generic.  However, in the meantime this works as a interim
solution.

Signed-off-by: Jake Moilanen <moilanen@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>