powerpc/powernv: Require MMU_NOTIFIER to fix NPU build

In the recent commit 1ab66d1f ("powerpc/powernv: Introduce address
translation services for Nvlink2") the NPU code gained a dependency on MMU
notifiers.

All our defconfigs have KVM enabled, which selects MMU_NOTIFIER, but if KVM is
not enabled then the build breaks.

Fix it by always selecting MMU_NOTIFIER when we're building powernv.

Fixes: 1ab66d1f ("powerpc/powernv: Introduce address translation services for Nvlink2")
Signed-off-by: Alistair Popple <alistair@popple.id.au>
[mpe: Reword change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>