IIO: Add a modifier for sqrt(x^2+y^2)

There will probably be a number of such modifiers eventually but
this one is used in the adis16204 accelerometer driver.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-core.c

@@ -70,6 +70,7 @@ static const char * const iio_modifier_names[] = {
 	[IIO_MOD_X] = "x",
 	[IIO_MOD_Y] = "y",
 	[IIO_MOD_Z] = "z",
+	[IIO_MOD_ROOT_SUM_SQUARED_X_Y] = "sqrt(x^2+y^2)",
 	[IIO_MOD_LIGHT_BOTH] = "both",
 	[IIO_MOD_LIGHT_IR] = "ir",
 };