MIPS: SGI-IP27: Move get_nasid() to a IP27 specific file

get_nasid() will be different for SGI-IP35, therefore move IP27
implementation to IP27 specific 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