iio: adc: Nothing in ADC should be a bool CONFIG

The whole IIO subsystem can be built as modules.  If you make it a
module then stuff marked as "Y" in the adc directory simply won't be
linked in properly.

The two configs that were wrong were EXYNOS_ADC and LP8788_ADC.  I
know for a fact that EXYNOS_ADC will work as a module. I assume
LP8788_ADC will also be fine.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>