iio: hi8435: make in_voltage_sensing_mode_available visible

Possible values of sensing_mode are encoded with strings and actual
strings used are not obvious.

Provide a hint by enabling in_voltage_sensing_mode_available attribute.
Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/hi8435.c

@@ -356,6 +356,7 @@ static const struct iio_enum hi8435_sensing_mode = {
 
 static const struct iio_chan_spec_ext_info hi8435_ext_info[] = {
 	IIO_ENUM("sensing_mode", IIO_SEPARATE, &hi8435_sensing_mode),
+	IIO_ENUM_AVAILABLE("sensing_mode", &hi8435_sensing_mode),
 	{},
 };