iio: ABI: add filter types for ad7173

Add new filter types used in the ad7173 driver to the IIO ABI
documentation. These chips have a few filter types that haven't been
seen before in other drivers.

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

Author: dlech

Documentation/ABI/testing/sysfs-bus-iio

@@ -2327,6 +2327,11 @@ Description:
 		  performance
 		* "sinc5+avg" - Sinc5 + averaging by 4.
 		* "sinc5+pf1" - Sinc5 + device specific Post Filter 1.
+		* "sinc5+sinc1" - Sinc5 + Sinc1.
+		* "sinc5+sinc1+pf1" - Sinc5 + Sinc1 + device specific Post Filter 1.
+		* "sinc5+sinc1+pf2" - Sinc5 + Sinc1 + device specific Post Filter 2.
+		* "sinc5+sinc1+pf3" - Sinc5 + Sinc1 + device specific Post Filter 3.
+		* "sinc5+sinc1+pf4" - Sinc5 + Sinc1 + device specific Post Filter 4.
 		* "wideband" - filter with wideband low ripple passband
 		  and sharp transition band.