MAINTAINERS: add Max Filippov as an xtensa port maintainer

Signed-off-by: Chris Zankel <chris@zankel.net>

MAINTAINERS

@@ -6802,6 +6802,8 @@ F:	drivers/char/tlclk.c
 
 TENSILICA XTENSA PORT (xtensa)
 M:	Chris Zankel <chris@zankel.net>
+M:	Max Filippov <jcmvbkbc@gmail.com>
+L:	linux-xtensa@linux-xtensa.org
 S:	Maintained
 F:	arch/xtensa/