MAINTAINERS: add xtensa irqchips to xtensa port entry

Now that irqchip drivers for xtensa live outside arch/xtensa we'd like
to add them to our maintenance list.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Marc Gauthier <marc@cadence.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>