power: ltc2941-battery-gauge: Disable continuous monitoring on shutdown

The driver sets the fuel gauge to continuous monitoring on startup, for
the models that support this. When the board shuts down, the chip remains
in that mode, causing a few mA drain on the battery every 2 or 10 seconds.

This patch registers a shutdown handler that turns off the monitoring to
prevent this battery drain.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>