MIPS: Ci20: Add support for rootfs on NFS to defconfig

Now that ethernet support is in the kernel, add the option to use a
rootfs over NFS to enable automated testing of upstream kernels on a
Ci20.

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Harvey Hunt <harveyhuntnexus@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17314/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>