MIPS: pistachio: Enable Root FS on NFS in defconfig

MIPS: pistachio: Enable Root FS on NFS in defconfig

When the upstream kernel pistachio_defconfig is built & tested on the
ci40 platform the current lack of these options leads to essentially
false failures when the RFS fails to mount.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: James Hartley <james.hartley@sondrel.com>
Cc: Ionela Voinescu <ionela.voinescu@imgtec.com>
Cc: James Hartley <james.hartley@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16763/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>