MAINTAINERS: Add maintenance information for psi

Add a maintainer section for psi, as it's a user-visible, configurable
kernel feature.

The patches are still routed through the scheduler tree due to the
close integration with that code, but get_maintainers.pl does the
right thing and makes sure everybody gets CCd:

$ ./scripts/get_maintainer.pl -f kernel/sched/psi.c
Johannes Weiner <hannes@cmpxchg.org> (maintainer:PRESSURE STALL INFORMATION (PSI))
Ingo Molnar <mingo@redhat.com> (maintainer:SCHEDULER)
Peter Zijlstra <peterz@infradead.org> (maintainer:SCHEDULER)
...

Reported-by: Ivan Babrou <ivan@cloudflare.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200316191333.115523-4-hannes@cmpxchg.org