ARM: imx_v6_v7_defconfig: Enable TOUCHSCREEN_AD7879

The Computer on module Colibri iMX7S/iMX7D has
an AD7879 touchscreen controller.

This patch enables the TOUCHSCREEN_AD7879 and TOUCHSCREEN_AD7879_I2C
which support the mentioned controller in i2c mode.

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