Documentation: devres: add missing mdio helper

We have a devres variant of mdiobus_register() but it's not listed in
devres.rst. Add it under other mdio devm functions.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -343,6 +343,7 @@ MDIO
   devm_mdiobus_alloc()
   devm_mdiobus_alloc_size()
   devm_mdiobus_free()
+  devm_mdiobus_register()
 
 MEM
   devm_free_pages()