rtc: abx80x: Configure reserved bits in RV1805

The RV1805 uses smaller package than the AB1805, discern those two
chips based on the compatible value and configure reserved bits in
the RV1805 to prevent current leakage and accidental test mode entry.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
To: linux-rtc@vger.kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>