powerpc/pseries: CONFIG_PSERIES_MSI should depend on PPC_PSERIES

Previously PSERIES_MSI depended on PPC_PSERIES via EEH. However in
commit 317f06de "powerpc/eeh: Move common part to kernel directory" we
made CONFIG_EEH selectable on POWERNV. That leaves us with PSERIES_MSI
being live even when PSERIES=n. Fix it by making PSERIES_MSI depend
directly on PPC_PSERIES.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Reviewed-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>