rtc: rv8803: Extend compatibility with the rx8900

The Seiko Epson's RTC RX8900 layout register is compatible with the
RV8803. So let's add its ID in order to reuse the same driver.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>