ath79: fix missing watchdog core dependency for hwmon-sch5627

Fixes following error uncovered while building ath79/tiny on 5.4:

 Package kmod-hwmon-sch5627 is missing dependencies for the following
 libraries:
 watchdog.ko

See the similar commit e546e625 ("malta: fix missing watchdog core
dependency for hwmon-sch5627") for a detailed explanation.

Signed-off-by: David Bauer <mail@david-bauer.net>

target/linux/ath79/config-5.4

@@ -244,3 +244,4 @@ CONFIG_TICK_CPU_ACCOUNTING=y
 CONFIG_TINY_SRCU=y
 CONFIG_USB_SUPPORT=y
 CONFIG_USE_OF=y
+CONFIG_WATCHDOG_CORE=y