Documentation: devres: add missing IO helper

Add missing devm_request_free_mem_region() to devres.rst.
It's introduced by commit 0092908d ("mm: factor out a
devm_request_free_mem_region helper").
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220927080215.1359979-1-yangyingliang@huawei.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/driver-model/devres.rst

@@ -301,6 +301,7 @@ IO region
   devm_release_region()
   devm_release_resource()
   devm_request_mem_region()
+  devm_request_free_mem_region()
   devm_request_region()
   devm_request_resource()