MAINTAINERS: update M68K entry

Add myself to be the M68K maintainer.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>

MAINTAINERS

@@ -133,7 +133,8 @@ F: include/hw/lm32/
 F: tests/tcg/lm32/
 
 M68K
-S: Orphan
+M: Laurent Vivier <laurent@vivier.eu>
+S: Maintained
 F: target-m68k/
 F: hw/m68k/