MAINTAINERS: Fix 'F' entry for the power supply class

For some reason the maintainers file only specifies power
supply core files.

We're surely interested in individual drivers as well, so fix
the entry.

Reported-by: Grazvydas Ignotas <notasas@gmail.com>
Reported-by: NeilBrown <neilb@suse.de>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>