rtc: rework rtc_register_device() resource management

rtc_register_device() is a managed interface but it doesn't use devres
by itself - instead it marks an rtc_device as "registered" and the devres
callback for devm_rtc_allocate_device() takes care of resource release.

This doesn't correspond with the design behind devres where managed
structures should not be aware of being managed. The correct solution
here is to register a separate devres callback for unregistering the
device.

While at it: rename rtc_register_device() to devm_rtc_register_device()
and add it to the list of managed interfaces in devres.rst. This way we
can avoid any potential confusion of driver developers who may expect
there to exist a corresponding unregister function.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201109163409.24301-8-brgl@bgdev.pl

drivers/rtc/rtc-sun4v.c

@@ -86,7 +86,7 @@ static int __init sun4v_rtc_probe(struct platform_device *pdev)
 	rtc->range_max = U64_MAX;
 	platform_set_drvdata(pdev, rtc);
 
-	return rtc_register_device(rtc);
+	return devm_rtc_register_device(rtc);
 }
 
 static struct platform_driver sun4v_rtc_driver = {