MIPS: ci20: Enable NAND and UBIFS support in defconfig.

Update the Ci20's defconfig to enable the JZ4780's NAND driver and
therefore access to the UBIFS rootfs.

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12699/


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