ARM: exynos_defconfig: Enable ChromeOS EC chardev driver

Exynos Chromebooks have an Embedded Controller known as the ChromeOS EC
Enable the driver that provides an interface to access from user-space.

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