Documentation: ABI: document IIO in_temp_filter_notch_center_frequency file

max31856 IIO driver introduced a new attribute
"in_temp_filter_notch_center_frequency".
This patch adds it to the list of documented ABI for sysfs-bus-iio

Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Patrick Havelange <patrick.havelange@essensium.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: Chuhong Yuan <hslester96@gmail.com>
Cc: Daniel Gomez <dagmcr@gmail.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1726,3 +1726,10 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		List of valid periods (in seconds) for which the light intensity
 		must be above the threshold level before interrupt is asserted.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_filter_notch_center_frequency
+KernelVersion:	5.5
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Center frequency in Hz for a notch filter. Used i.e. for line
+		noise suppression.