rtc: rx8010: convert to using regmap

This driver requires SMBUS to work. We can relax this requirement if we
switch to using i2c regmap and let the regmap sub-system figure out how
to talk to the bus.

This also has the advantage of shrinking the code for register updates.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200914154601.32245-14-brgl@bgdev.pl