dt-bindings: rtc: Add RV1805 to abracon,abx80x bindings

Add compatible string for Microcrystal RV1805 RTC, which is compatible
with AB1805. The RV1805 uses smaller 10 pin package, while the AB1805
uses larger 16pin package, however the die inside the chip is the same.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Rob Herring <robh+dt@kernel.org>
To: devicetree@vger.kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>