rtc: ds1343: use rtc_add_group

Use rtc_add_group to add the sysfs group in a race free manner.
This has the side effect of moving the files to their proper location.

Link: https://lore.kernel.org/r/20191019204941.6203-4-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>