gpio: Add missing device-managed documentation

Add the GPIO-related device-managed functions to the list of functions
in Documentation/driver-model/devres.txt.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-model/devres.txt

@@ -308,3 +308,8 @@ SLAVE DMA ENGINE
 
 SPI
   devm_spi_register_master()
+
+GPIO
+  devm_gpiod_get()
+  devm_gpiod_get_index()
+  devm_gpiod_put()