power: supply: core: convert to fwnnode

Replace any DT specific code with fwnode in the power-supply
core.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Hans de Goede <hansg@kernel.org>
Link: https://lore.kernel.org/r/20250430-psy-core-convert-to-fwnode-v2-4-f9643b958677@collabora.com


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>