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

MAINTAINERS

@@ -15504,6 +15504,7 @@ F:	drivers/net/ppp/pptp.c
 
 PRESSURE STALL INFORMATION (PSI)
 M:	Johannes Weiner <hannes@cmpxchg.org>
+M:	Suren Baghdasaryan <surenb@google.com>
 S:	Maintained
 F:	include/linux/psi*
 F:	kernel/sched/psi.c