MIPS: SGI-IP27: get rid of asm/sn/sn0/ip27.h

The only piece needed in ip27.h is SEND_NMI. Move the define to the only
place where it's used and remove ip27.h

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