rtc: add ht1380 driver

Support Holtek HT1380/HT1381 Serial Timekeeper Chip. It provides seconds
, minutes, hours, day of the week, date, month and year information.

Datasheet:
https://www.holtek.com.tw/documents/10179/11842/ht1380_1v130.pdf



Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>