rtc: ds1307: add offset sysfs for mt41txx chips.

m41txx chips can hold a calibration value to get correct clock bias.

Add offset handling (ranging between -63ppm and 126ppm) via sysfs.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>