[C25] disable stm32 watchdog on boot

Stop watchdog timer on STM32 chip when U-Boot boots

In the previous MR (linux-seco-imx!214), a watchdog driver for STM32 chip was introduced. But when the watchdog triggers in Linux and the hardware resets, the watchdog is not stopped by U-Boot. As a result, when a watchdog timeout will expire, the board will be rebooted again, and again. To prevent this infinite loop, introduced an init routine to stop watchdog at boot stage.

And in case, if watchdog will be stopped (or periodically pinged) after OS(Linux) is booted, introduced some example code how to set a watchdog timeout to desire value and trigger watchdog in U-Boot. This routine is not enabled at the moment.