MIPS: Define ST0_NMI in asm/mipsregs.h

This is used by the forthcoming OCTEON watchdog patch.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
To: wim@iguana.be
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1498/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>