watchdog: imgdpc: select WATCHDOG_CORE

The imgpdc_wdt driver can be built on all architectures with
CONFIG_COMPILE_TEST, but fails if no other watchdog driver is
enabled:

drivers/watchdog/built-in.o: In function `pdc_wdt_remove':
imgpdc_wdt.c:(.text+0x74): undefined reference to `watchdog_unregister_device'

This adds the normal 'select WATCHDOG_CORE' that is needed to
ensure the driver always builds cleanly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>