dt-bindings: rtc: ds1307: add rx8130 aux-voltage-chargeable support

Epson's RX8130 was not charged before. A related patch will allow
optional charging.

Signed-off-by: Bastian Krause <bst@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200917183246.19446-4-bst@pengutronix.de