ARM: imx_v6_v7_defconfig: cleanup with savedefconfig

Generate imx_v6_v7_defconfig by doing:

make imx_v6_v7_defconfig
make savedefconfig
cp defconfig arch/arm/configs/imx_v6_v7_defconfig

No functional change. The goal here is to cleanup imx_v6_v7_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>