power: supply: axp288_charger: Register extcon notifers after power_supply

The extcon notifier work calls power_supply_changed on the power_supply
we register, so the extcon notifiers should be registered after we
register the power_supply.

While touching this code anyways, refactor the code for the 3 cable types
into a loop to avoid code repetition.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>