[POWERPC] Enable MSI mappings for MPIC

On some Apple machines the HT MSI mappings are not enabled by firmware, so
we need to do it by hand.

We can't use the pci routines as this code runs too early.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>