power/ltc2941-battery-gauge.c: Use the devicetree node name as supply name

Make it possible to set the name of the supply from the devicetree.
Like other power supply drivers just use the node name. This makes
the code smaller as well, as it doesn't need to allocate memory to
hold the name and allocate a unique ID.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Sebastian Reichel <sre@kernel.org>