Documentation: devres: add missing MEM helper

Documentation: devres: add missing MEM helper

Add missing devm_kstrdup_const() to devres.rst. It's introduce by
commit 09d1ea1c7309 ("devres: provide devm_kstrdup_const()").

Fixes: 09d1ea1c7309 ("devres: provide devm_kstrdup_const()")
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20221102025534.1450337-1-yangyingliang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>