power: supply: core: fix build of extension sysfs group if CONFIG_SYSFS=n

power: supply: core: fix build of extension sysfs group if CONFIG_SYSFS=n

Add and use wrapper functions for the sysfs interaction.
Restore the compatibility of CONFIG_POWER_SUPPLY=y and CONFIG_SYSFS=n.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/lkml/20241218195229.GA2796534@ax162/
Fixes: 288a2cabcf6b ("power: supply: core: add UAPI to discover currently used extensions")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20241219-psy-extensions-sysfs-v1-1-868fc6cb46d6@weissschuh.net
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>