ARM: mxs_defconfig: Cleanup mxs_defconfig

Generate mxs_defconfig by doing:

make mxs_defconfig
make savedefconfig
cp defconfig arch/arm/configs/mxs_defconfig

No functional change. The goal here is to cleanup mxs_defconfig file to
make easier and cleaner the addition of new entries.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>