rtc: define RTC_VL_READ values

Currently, the meaning of the value returned by RTC_VL_READ is undocumented
and left to the driver implementation. In order to get more meaningful
values, define a set of values to use as to make clear to userspace what is
the status of the various voltages feeding the RTC.

Link: https://lore.kernel.org/r/20191214220259.621996-2-alexandre.belloni@bootlin.com


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