MIPS: IP27: Fix clash with NMI_OFFSET from hardirq.h

There was already a define for NMI_OFFSET in asm/sn/addr.h, which now
clashes with linux/hardirq.h. Rename the one in sn/addr.h to fix IP27
builds..

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>