MIPS: Provide fallback reboot/poweroff/halt implementations

If a machine-specific hook is not implemented for restart, poweroff,
or halt, fall back to halting secondary CPUs, disabling interrupts,
and spinning.  In the case of restart, attempt to restart the system
via do_kernel_restart() (which will call any registered restart
handlers) before halting.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9600/


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