watchdog: da9055_wdt needs to select WATCHDOG_CORE

DA9055_WATCHDOG (introduced in v3.8) needs to select WATCHDOG_CORE so that it will
build cleanly.  Fixes these build errors:

da9055_wdt.c:(.text+0xe9bc7): undefined reference to `watchdog_unregister_device'
da9055_wdt.c:(.text+0xe9f4b): undefined reference to `watchdog_register_device'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Dajun Chen <dchen@diasemi.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-watchdog@vger.kernel.org
Cc: stable <stable@vger.kernel.org>