watchdog: cadence: Add dependency on HAS_IOMEM

Not all architectures have io memory.

Fixes:
drivers/built-in.o: In function `cdns_wdt_probe':
cadence_wdt.c:(.text+0x33b7c9): undefined reference to `devm_ioremap_resource'

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>