iio: adc: Kconfig: exynos_adc depends on CONFIG_OF

As the exynos_adc driver only supports device tree registration.
Making driver depend on CONFIG_OF solves possible errors during probe.

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>