rtc: add rtc-abx80x, a driver for the Abracon AB x80x i2c rtc

This is a basic driver for the ultra-low-power Abracon AB x80x series
of RTC chips. It supports in particular, the supersets AB0805 and AB1805.
It allows reading and writing the time, and enables the supercapacitor or
battery charger.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>