rtc: rv8803: Add support for the Epson RX8804 RTC

The Epson RX8804 RTC has the same programming model as RV8803.

Add support for it in the driver.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211130125830.1166194-2-festevam@gmail.com