MAINTAINERS: Pick up power supply maintainership

Anton stated that he would have to abandon power supply maintainership
due to the lack of time. By agreement with him and David, pick up power
supply tree.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Anton Vorontsov <anton@enomsg.org>