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: Jiri Kosina <jkosina@suse.cz>

Documentation/driver-model/devres.txt

@@ -237,6 +237,10 @@ MEM
   devm_kzalloc()
   devm_kfree()
 
+IIO
+  devm_iio_device_alloc()
+  devm_iio_device_free()
+
 IO region
   devm_request_region()
   devm_request_mem_region()