m68k/mac: Avoid soft-lockup warning after mach_power_off

Disable interrupts for power-off, like other platforms do. This prevents
meaningless warnings from the soft-lockup detector on models with a power
supply unit not under software control.

Reported-by: Stan Johnson <userm57@yahoo.com>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>