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>