rtc: rv3029: add RTC_VL_READ and RTC_VL_CLEAR support

The RV3029 can report three different conditions: power on, voltage dropped
and data is lost and voltage is low and temperature compensation has
stopped. The first two conditions amount to the same status, the RTC data
is invalid.

VLOW1 has to be cleared manually to resume temperature compensation, this
is achieved using RTC_VL_CLEAR.

Link: https://lore.kernel.org/r/20191214221022.622482-11-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>