ARM: dts: Add trackpad to exynos5250-spring

The HP Chromebook 11 uses an Atmel maXTouch as trackpad.
The keymap was found by trial-and-error.

Signed-off-by: Andreas Faerber <afaerber@suse.de>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>