iio: adc: add exynos adc driver under iio framwork

This patch adds New driver to support:
1. Supports ADC IF found on EXYNOS4412/EXYNOS5250
   and future SoCs from Samsung
2. Add ADC driver under iio/adc framework
3. Also adds the Documentation for device tree bindings

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>