MAINTAINERS: Add Peter explicitly to the psi section

Peter is kind enough to route the low-volume psi patches through the
scheduler tree, but he is frequently not CC'd on them.

While he is matched through the SCHEDULER maintainers and reviewers on
kern/sched/*, that list is long, and mostly not applicable to psi
code. Thus, patch submitters often just CC the explicit PSI entries.

Add him to that section, to make sure he gets those patches.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230801133235.GA1766885@cmpxchg.org