ARM: exynos: dts: cros5250: add EC device

Add basic EC information to device tree, currently only describing the
keyboard and keymap.

Reviewed-by: Doug Anderson <dianders@chromium.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Olof Johansson <olof@lixom.net>