MIPS: DEC: Remove the Halt button interrupt on R4k systems

MIPS: DEC: Remove the Halt button interrupt on R4k systems

On R4k DECstations the Halt button is wired to the NMI processor input
rather than an ordinary interrupt input such as on R3k DECstations.  This
is possible with a different design of the CPU daughtercard that routes
the Halt button line from the baseboard connector.  Additionally the
interrupt input has been reused for a different purpose on the KN04 and
KN05 R4k CPU daughtercards so it is better kept masked.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6705/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>