drivers: watchdog: wdt_dw: resetting CPU or SoC is not configurable

specific to platform, watchdog reset line can be connected either to
CPU, SOC or none of the entity. These resets cannot be configured from the
application. So added a warning message when application configures this
option

Signed-off-by: Balsundar Ponnusamy <balsundar.ponnusamy@intel.com>