rtc: rx6110: add i2c support

The RX6110 also supports I2C, so this patch adds support for it to the
driver.

This also renames the SPI specific functions and variables to include
`_spi_` in their names.

Signed-off-by: Claudius Heine <ch@denx.de>
Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201117121817.953924-3-ch@denx.de