IIO: Add a modifier for x^2+y^2+z^2

There will probably be a number of such modifiers eventually but
this one is used in the adis16240 accelerometer driver.
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-core.c

@@ -71,6 +71,7 @@ static const char * const iio_modifier_names[] = {
 	[IIO_MOD_Y] = "y",
 	[IIO_MOD_Z] = "z",
 	[IIO_MOD_ROOT_SUM_SQUARED_X_Y] = "sqrt(x^2+y^2)",
+	[IIO_MOD_SUM_SQUARED_X_Y_Z] = "x^2+y^2+z^2",
 	[IIO_MOD_LIGHT_BOTH] = "both",
 	[IIO_MOD_LIGHT_IR] = "ir",
 };

include/linux/iio/types.h

@@ -45,6 +45,7 @@ enum iio_modifier {
 	IIO_MOD_LIGHT_BOTH,
 	IIO_MOD_LIGHT_IR,
 	IIO_MOD_ROOT_SUM_SQUARED_X_Y,
+	IIO_MOD_SUM_SQUARED_X_Y_Z,
 };
 
 #define IIO_VAL_INT 1