watchdog: ts72xx_wdt: fix build error

watchdog: ts72xx_wdt: fix build error

If TS72XX_WATCHDOG is y and WATCHDOG_CORE is not enabled or its m,
then building fails:

drivers/watchdog/ts72xx_wdt.o: in function `ts72xx_wdt_probe':
	ts72xx_wdt.c:(.text+0x14c): undefined reference to \
	`watchdog_init_timeout'
	ts72xx_wdt.c:(.text+0x15c): undefined reference to \
	`devm_watchdog_register_device'

Select WATCHDOG_CORE to fix this.

Signed-off-by: Shyam Saini <shyam.saini@savoirfairelinux.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200406215008.30468-1-shyam.saini@savoirfairelinux.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>