power: supply: sbs-battery: add support for time_to_empty_now attribute

As defined by the Smart Battery Data Specification.

An _AVG suffix is added to the enum values REG_TIME_TO_EMPTY and
REG_TIME_TO_FULL to make the distinction clear.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>