ARM: exynos_defconfig: Enable NTC Thermistors support

The Exynos5420 Peach Pit and Exynos5800 Peach Pi Chromebooks have
IIO based ADC thermistors. Enable built-in support for its driver.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>