rtc: abx80x: Add Device Tree matching table

Enable automatic loading of the module when a Device Tree overlay
specifies a device supported by this driver.

Signed-off-by: Kevin P. Fleming <kevin+linux@km6g.us>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200528114617.166587-1-kevin+linux@km6g.us