drivers: watchdog: Adding watchdog support for sam SOC

Adding watchdog driver for sam SoC. The current implemntation just
diables the watchdog on device boot.
This PR adds the following support for SAM3X, SAM4S and SAME70
1. Activate processor reset
2. Activate all reset
3. Generate interrupt on watchdog timer expiration.

Signed-off-by: Subramanian Meenakshi Sundaram <subbu147@gmail.com>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>