MAINTAINERS: csky: Add mailing list for csky

Add mailing list and it's convenient for maintain C-SKY
subsystem.
Signed-off-by: NGuo Ren <guoren@linux.alibaba.com>

MAINTAINERS

@@ -3649,6 +3649,7 @@ F:	sound/pci/oxygen/
 
 C-SKY ARCHITECTURE
 M:	Guo Ren <guoren@kernel.org>
+L:	linux-csky@vger.kernel.org
 T:	git https://github.com/c-sky/csky-linux.git
 S:	Supported
 F:	arch/csky/