dt/bindings: rtc: rx8900: Add an entry for RX8804

The Epson RX8804 RTC has the same programming model as RV8803 and
RX8900.

Add an entry for it in the binding document.

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/20211130125830.1166194-1-festevam@gmail.com