Documentation: list RTC devres helpers in devres.rst

It's customary to list all devres helpers in devres.rst. Add missing RTC
routines.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201109163409.24301-4-brgl@bgdev.pl

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

@@ -411,6 +411,10 @@ RESET
   devm_reset_control_get()
   devm_reset_controller_register()
 
+RTC
+  devm_rtc_device_register()
+  devm_rtc_allocate_device()
+
 SERDEV
   devm_serdev_device_open()