rtc: pcf85063: add offset correction support

The PCF850363 has an offset correction with two modes:

With mode 0, the correction is triggered once every two hours and then
correction pulses are applied once per minute until the programmed
correction values have been implemented. This gives a step of 4.34 ppm.

With mode 1, the correction is triggered once every four minutes and then
correction pulses are applied once per second up to a maximum of 60 pulses.
When correction values greater than 60 pulses are used, additional
correction pulses are made in the 59 th second. This gives a step of 4.069
ppm.

Use the correction closest to the requested value.

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