MIPS: configs: ci20: Enable ext4

Now that we have MMC support, enable ext2/3/4 support
in the CI20 defconfig.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Acked-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>