[POWERPC] Support setting affinity for U3/U4 MSI sources

Hook up affinity-setting for U3/U4 MSI interrupt sources.

Tested on Quad G5 with myri10ge.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>