rtc: Kconfig: select REGMAP_I2C when necessary

Some i2c RTC drivers are using regmap but are not selecting REGMAP_I2C
which may lead to build failures.

Link: https://lore.kernel.org/r/20200127221724.10160-2-alexandre.belloni@bootlin.com


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