watchdog: wdat_wdt: Select WATCHDOG_CORE

The WDAT watchdog driver uses functionality provided by the watchdog timer
core but it did not select it explicitly. This results following linker
error when only WDAT_WDT is enabled in Kconfig:

  drivers/built-in.o: In function `wdat_wdt_probe':
  drivers/watchdog/wdat_wdt.c:444: undefined reference to `devm_watchdog_register_device'

Fix this by explicitly selecting WATCHDOG_CORE when WDAT watchdog driver is
enabled.

Fixes: 058dfc767008 (ACPI / watchdog: Add support for WDAT hardware watchdog)
Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>