iio: adc: Document the regulator/clocks for exynos-adc

The exynos ADC won't work without a regulator called "vdd" and a clock
called "adc".  Document this fact in the device tree bindings.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>