ARM: imx_v6_v7_defconfig: Enable TOUCHSCREEN_ATMEL_MXT

Toradex iMX6/iMX7-based modules and boards support LCDs
with an MicroChip ATMXT1066T2 touchscreen controller.

This patch enables the TOUCHSCREEN_ATMEL_MXT which supports
MXT-series controllers, including the ATMXT1066T2.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>