[POWERPC] MPIC MSI allocator

To support MSI on MPIC we need a way to reserve and allocate hardware irq
numbers, this patch implements an allocator for that purpose.

New firmware platforms must define a "msi-available-ranges" property on their
MPIC node for MSI to work. For U3/U4 we do a best-guess setup.

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