powerpc/configs: Enable PPC_UV in powernv_defconfig

Make sure the ultravisor code at least gets some build testing by
enabling it in powernv_defconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220929051517.1903079-1-mpe@ellerman.id.au