iio: ph: add IIO_PH channel type

Signed-off-by: Matt Ranostay <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio

···
 		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:	[e-mail]
+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.

drivers/iio/industrialio-core.c

···
 	[IIO_VELOCITY] = "velocity",
 	[IIO_CONCENTRATION] = "concentration",
 	[IIO_RESISTANCE] = "resistance",
+	[IIO_PH] = "ph",
 };
 
 static const char * const iio_modifier_names[] = {

include/uapi/linux/iio/types.h

···
 	IIO_VELOCITY,
 	IIO_CONCENTRATION,
 	IIO_RESISTANCE,
+	IIO_PH,
 };
 
 enum iio_modifier {