MAINTAINERS: power: supply: cover also header files

Only one header (smartreflex.h) in include/linux/power/ is not related
to battery or charger drivers.  All others should be covered by power
supply maintainers entry so patches to them will reach respective people
and lists.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>