bcm63xx: fix missing watchdog core dependency

bcm63xx: fix missing watchdog core dependency

This patch follows the other patches that added the watchdog
core to various (armvirt, malta, ath79, ...) targets that
have been hit by the following build error:

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

In theory, we could have just added the CONFIG_WATCHDOG_CORE=y
to the Kconfig variable of kmod-hwmon-sch5627's package definition.
This would have forced the watchdog core to be builtin and less
architectures would need to be updated. But we might as well follow
through here.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>