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