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>

drivers/iio/adc/Kconfig

@@ -105,6 +105,7 @@ config AT91_ADC
 
 config EXYNOS_ADC
 	bool "Exynos ADC driver support"
+	depends on OF
 	help
 	  Core support for the ADC block found in the Samsung EXYNOS series
 	  of SoCs for drivers such as the touchscreen and hwmon to use to share