iio: adc: ad7768-1: add multiple scan types to support 16-bits mode

iio: adc: ad7768-1: add multiple scan types to support 16-bits mode

When the device is configured to decimation x8, only possible in the
sinc5 filter, output data is reduced to 16 bits in order to support
1 MHz of sampling frequency due to clock limitation.

Use multiple scan types feature to enable the driver to switch
scan type at runtime, making it possible to support both 24-bit and
16-bit resolution.

Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/08780fd4a59885f1f250759ce655420bd1dbb383.1749569957.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>