hw/arm/stellaris: Implement watchdog timer

Implement the watchdog timer for the stellaris boards.
This device is a close variant of the CMSDK APB watchdog
device, so we can model it by subclassing that device and
tweaking the behaviour of some of its registers.

Signed-off-by: Michel Heily <michelheily@gmail.com>
Reviewed-by: Peter Maydell <petser.maydell@linaro.org>
[PMM: rewrote commit message, fixed a few checkpatch nits,
 added comment giving the URL of the spec for the Stellaris
 variant of the watchdog device]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>