ARM: imx_v6_v7_defconfig: Remove CONFIG_SOC_LS1021A

imx_v6_v7_defconfig is not suitable for booting LS1021 as it needs
LPAE.

Remove this option.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>