rtc: isl1208: avoid possible sysfs race

Use rtc_add_group to add the common sysfs group to avoid a possible race
condition.

[Denis.Osterland@diehl.com: use to_i2c_client(dev->parent)]
Signed-off-by: Denis Osterland <Denis.Osterland@diehl.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

The move of atrim, dtrim usr sysfs properties from i2c device
to rtc device require to access them via dev->parent.
This patch also aligns timestamp0.