[POWERPC] Tell Phyp we support MSI

Tell Phyp we support MSI via the client architecture support mechanism.

Signed-off-by: Jake Moilanen <moilanen@austin.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>