rtc: rzn1: Add oscillator offset support

The RZN1 RTC can compensate the imprecision of the oscillator up to
approximately 190ppm.

Seconds can last slightly shorter or longer depending on the
configuration.

Below ~65ppm of correction, we can change the time spent in a second
every minute, which is the most accurate compensation that the RTC can
offer. Above, the compensation will be active every 20s.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220516082504.33913-5-miquel.raynal@bootlin.com