rtc: ds3232: convert to use regmap

This is preparation for merging rtc-ds3232 i2c driver and rtc-ds3234
spi driver.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Dennis Aberilla <denzzzhome@yahoo.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>