[hardware_watchdog] add "missing" watchdog_disable()

[tjko]

Currently SDK doesn't provide a way do disable watchdog after it's been enabled using watchdog_enable().

Please consider adding "watchdog_disable()" to easily disable watchdog.

Maybe something along the lines:

void watchdog_disable()
{
        hw_clear_bits(&watchdog_hw->ctrl, WATCHDOG_CTRL_ENABLE_BITS);
}

[peterharperuk]

watchdog_disable has been added