drivers: wdt_tco: Use DT_DRV_COMPAT interface

DT_DRV_COMPAT was unused for this driver. To get a node identifier
DT_NODELABEL() macro was used. Refactor to use the correct interfaces
using instance number 0.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>