ARM: exynos_defconfig: Enable LM90 driver

multi_v7_defconfig has it as Y already, so build it in here, too, for
consistency, and therefore build in HWMON as well.

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