MIPS: PowerTV: Remove mips_machine_halt()

mips_machine_halt() is same as mips_machine_restart().  Also delete the
registration of _machine_halt and pm_power_off because mips_machine_halt()
is the restart function.

Signed-off-by: Yoichi Yuasa <yuasa@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/798/
Reviewed-by: David VomLehn <dvomlehn@cisco.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>