iio: Add I/Q modifiers

I/Q modifiers can be used to denote signals which are represented by a
in-phase and a quadrature component.

The ABI documentation describes the I and Q modifiers for current and
voltage channels for now as those will be the most likely users.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/uapi/linux/iio/types.h

@@ -70,6 +70,8 @@ enum iio_modifier {
 	IIO_MOD_WALKING,
 	IIO_MOD_STILL,
 	IIO_MOD_ROOT_SUM_SQUARED_X_Y_Z,
+	IIO_MOD_I,
+	IIO_MOD_Q,
 };
 
 enum iio_event_type {