iio:adc/dac:Kconfig: Update to drop OF dependencies.

We could probably drop a lot more of these, but for now this removes
unnecessary restrictions on stand alone ADC devices.

For these 3 drivers the false dependency seems to date all
the way back to their initial introduction.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>