dts: bindings: move ADC devicetree bindings to top level

Move the devicetree bindings for Analog-to-Digital Converters (ADCs)
from dts/bindings/iio/adc to dts/bindings/adc as Zephyr does not have
an IIO layer.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>