MAINTAINERS: Add AXP288 PMIC entry

Add an entry for the AXP288 PMIC drivers with myself as maintainer.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>