ARM: exynos_defconfig: Enable Atmel maXTouch support

Many Exynos based Chromebooks have an Atmel trackpad so enable
support for it by default will make easier for users.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>