iio: ph: add IIO_PH channel type

Signed-off-by: Matt Ranostay <[email protected]>
Signed-off-by: Jonathan Cameron <[email protected]>
diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
index 8fadd27..80c6fce 100644
--- a/Documentation/ABI/testing/sysfs-bus-iio
+++ b/Documentation/ABI/testing/sysfs-bus-iio
@@ -1493,3 +1493,10 @@
 		This ABI is especially applicable for humidity sensors
 		to heatup the device and get rid of any condensation
 		in some humidity environment
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_ph_raw
+KernelVersion:	4.5
+Contact:	[email protected]
+Description:
+		Raw (unscaled no offset etc.) pH reading of a substance as a negative
+		base-10 logarithm of hydrodium ions in a litre of water.