MAINTAINERS: add maillist information for LoongArch

Now there is a dedicated maillist ([e-mail]) for
LoongArch, add it for better collaboration.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Huacai Chen <[e-mail]>
Reviewed-by: WANG Xuerui <[e-mail]>
Cc: Huacai Chen <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Xuefeng Li <[e-mail]>
Cc: Guo Ren <[e-mail]>
Cc: Xuerui Wang <[e-mail]>
Cc: Jiaxun Yang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 LOONGARCH
 M:	Huacai Chen <[e-mail]>
 R:	WANG Xuerui <[e-mail]>
+L:	[e-mail]
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson.git
 F:	arch/loongarch/