rtc: s35390a: convert to devm_rtc_allocate_device

This allows further improvement of the driver and removes the need to
forward declare s35390a_driver.

Link: https://lore.kernel.org/r/20191016200848.30246-1-alexandre.belloni@bootlin.com


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