MAINTAINERS: add maillist information for LoongArch

Now there is a dedicated maillist (loongarch@lists.linux.dev) for
LoongArch, add it for better collaboration.

Link: https://lkml.kernel.org/r/20220616121456.3613470-1-chenhuacai@loongson.cn
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Xuefeng Li <lixuefeng@loongson.cn>
Cc: Guo Ren <guoren@kernel.org>
Cc: Xuerui Wang <kernel@xen0n.name>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>