watchdog: imx2_wdt: add restart handler support

  Register the watchdog as the system restart function
to the new introducing kernel restart call chain in the
driver instead of providing the restart in machine desc.
  This restart handler function is from the mxc_restart()
in arch/arm/mach-imx/system.c

Signed-off-by: Jingchang Lu <jingchang.lu@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>