watchdog: pnx4008: add restart handler

Add restart handler capability to the driver;
the restart handler implementation was taken from
"mach-lpc32xx" ("lpc23xx_restart" function).

Signed-off-by: Sylvain Lemieux <slemieux@tycoint.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>