rtc: rx8010: select REGMAP_I2C

The rtc-rx8010 uses the I2C regmap but doesn't select it in Kconfig so
depending on the configuration the build may fail. Fix it.

Signed-off-by: Yu-Tung Chang <mtwget@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210830052532.40356-1-mtwget@gmail.com