Documentation: devres: Add missing devm_kstrdup() managed interface

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-model/devres.txt

@@ -298,6 +298,7 @@ MEM
   devm_kmalloc()
   devm_kmalloc_array()
   devm_kmemdup()
+  devm_kstrdup()
   devm_kzalloc()
 
 PCI