dt-bindings: rtc: add rx8571 compatible

The Epson rx8571 is compatible with the Epson rx8581 RTC.

The difference between the rx8571 and rx8581 RTC is that the former has
16 bytes more RAM. This renders them incompatible from a DT point of view.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>