iio: adc: ad7124: add debugfs to disable single cycle mode

iio: adc: ad7124: add debugfs to disable single cycle mode

Add a boolean debugfs attribute to allow disabling the SINGLE_CYCLE
bit in the FILTER registers.

This causes data to be read on every conversion instead of doing the
usual 3 or 4 conversions per sample (depending on the filter). This is
only needed for very specific use cases, such as validating the
performance of the ADC. So we just expose this feature through debugfs
for the rare cases where it is needed by people who really know what
they are doing.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>