regulator: act8865-regulator: switch psy_cfg from of_node to fwnode

In order to remove .of_node from the power_supply_config struct,
use .fwnode instead.

Link: https://lore.kernel.org/r/20250430-psy-core-convert-to-fwnode-v2-1-f9643b958677@collabora.com


Reviewed-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>