IIO: hw_consumer: add devm_iio_hw_consumer_alloc

Add devm_iio_hw_consumer_alloc function that calls iio_hw_consumer_free
when the device is unbound from the bus.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>