watchdog: ni903x_wdt: Add NI 903x/913x watchdog driver

Add support for the watchdog timer on NI cRIO-903x and cDAQ-913x real-
time controllers.

Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>