powerpc/configs: Add BLK_DEV_NVME to pseries_defconfig

I've forgotten to manually enable NVME when building pseries kernels
for machines with NVME adapters. Since it's a reasonably common
configuration, enable it by default.

Signed-off-by: Anton Blanchard <anton@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200729040828.2312966-1-anton@ozlabs.org