Input: axp20x-pek - add module alias

Add a proper module alias so the driver can be autoloaded when the
parent axp20x mfd driver registers its cells.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>