arch: mips: update references to current linux-mips list

The linux-mips mailing list now lives at kernel.org. Update all references
in the kernel tree.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>