iio:filter:admv8818: Add sysfs ABI documentation

Add initial ABI documentation for admv8818 filter sysfs interfaces.

Signed-off-by: Antoniu Miclaus <[email protected]>
Signed-off-by: Jonathan Cameron <[email protected]>

Author: amiclaus

Documentation/ABI/testing/sysfs-bus-iio-filter-admv8818

@@ -0,0 +1,16 @@
+What:		/sys/bus/iio/devices/iio:deviceX/filter_mode_available
+KernelVersion:
+Contact:	[email protected]
+Description:
+		Reading this returns the valid values that can be written to the
+		on_altvoltage0_mode attribute:
+
+		- auto -> Adjust bandpass filter to track changes in input clock rate.
+		- manual -> disable/unregister the clock rate notifier / input clock tracking.
+
+What:		/sys/bus/iio/devices/iio:deviceX/filter_mode
+KernelVersion:
+Contact:	[email protected]
+Description:
+		This attribute configures the filter mode.
+		Reading returns the actual mode.