DT: watchdog: Add NXP LPC18xx Watchdog Timer binding documentation

Add the devicetree binding document for NXP LPC18xx Watchdog Timer.

Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>