rtc: isl12022: switch to using regmap API

rtc: isl12022: switch to using regmap API

The regmap abstraction allows us to avoid the private i2c transfer
helpers, and also offers some nice utility functions such as the
regmap_update_bits family.

While at it, simplify the code even more by not keeping track of
->write_enabled: rtc_set_time is not a hot path, so one extra i2c read
doesn't hurt (regmap_update_bits elides the write when the bits are
already as desired).

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20220921114624.3250848-9-linux@rasmusvillemoes.dk
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>