watchdog: sp5100_tco.c: fix potential build failure

isp5100_tco.c  uses watchdog core functions (from watchdog_core.c) and, when
compiled without CONFIG_WATCHDOG_CORE being set, it produces the
following build error:

ERROR: "devm_watchdog_register_device" [drivers/watchdog/sp5100_tco.ko] undefined!
ERROR: "watchdog_init_timeout" [drivers/watchdog/sp5100_tco.ko] undefined!

Fix this by selecting CONFIG_WATCHDOG_CORE.

Fixes: 7cd9d5ff ("watchdog: sp5100_tco: Convert to use watchdog subsystem")
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>