MIPS: SGI-IP27: move IP27 specific macro to IP27 specific header file

Extracting node id from HUB register is specific to IP27 alias SN0.
Move the macro definition to a SN0 header file.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org