xtensa: enable sorting extable at build time

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: David Daney <ddaney.cavm@gmail.com>
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>