rtc: ds1307: factor out century bit handling

The driver has lots of places with chip-specific code what doesn't
necessarily facilitate maintenance.

Let's describe chip-specific differences in century bit handling
in struct chip_desc to improve this.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>