Documentation: devres: add IIO device alloc/free functions to list

Add devm_iio_device_alloc() and devm_iio_device_free() functions
to list of supported calls.

Signed-off-by: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>