rtc: abx80x: Add utility function for writing configuration key

Writing one of key two values into the configuration key register
is a common operation, so a utility function has been added to
provide consistent behavior and eliminate code duplication.

Signed-off-by: Kevin P. Fleming <kevin+linux@km6g.us>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: linux-rtc@vger.kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200615105113.57770-2-kevin+linux@km6g.us