Do soft-power-on-reset in sys_reboot system call.

How to test this patch
  Execute "sudo reboot" from command line. The system will start shutdown
  sequence and finally reboot and U-Boot restarts.

Cc: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
Signed-off-by: Simon Horman <horms@verge.net.au>