rtc: ds1307: forward declare chips array instead of a bunch of functions

There used to be 16 declarations for static functions. By just adding a
declaration for the chips array and reordering the functions the 16
function declarations can be dropped.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>