[MIPS] TXx9: Cleanup restart/halt/power_off

Unify machine_restart/machine_halt/pm_power_off and add fallback
machine_halt routine.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>