MAINTAINERS: add Suren as psi co-maintainer

Suren wrote the poll() interface, which is a significant part of the
psi code and represents a large user of psi itself (Android). It's a
good idea to have him look at psi patches as well, and it's good to
have two people following things in case one of us is traveling.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220117120317.1581315-1-hannes@cmpxchg.org