iio: adc: exynos: describe drivers in KConfig

Describe better which driver applies to which SoC, to make configuring
kernel for Samsung SoC easier.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210924133515.112357-1-krzysztof.kozlowski@canonical.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>