Introduce power_supply_for_each_device(), which is a wrapper
for class_for_each_device() using the power_supply_class and
going through all devices.

This allows making the power_supply_class itself a local
variable, so that drivers cannot mess with it and simplifies
the code slightly.
Reviewed-by: Ricardo B. Marliere <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240301-psy-class-cleanup-v1-1-aebe8c4b6b08@collabora.comSigned-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>