MAINTAINERS: Add entry for X-Powers AXP family PMIC drivers

Add an entry for X-Powers AXP family PMIC drivers and list myself
as maintainer.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>