MAINTAINERS: add entry for freezer

Now that people are using freezer for non-suspend/hibernation stuff, it
should have separate maintainers entry so that it is easier to find.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>