watchdog: dw_wdt: Add dependency on HAS_IOMEM

On archs like S390 or um this driver cannot build nor work.
Make it depend on HAS_IOMEM to bypass build failures.

drivers/built-in.o: In function `dw_wdt_drv_probe':
drivers/watchdog/dw_wdt.c:302: undefined reference to `devm_ioremap_resource'
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/Kconfig

@@ -223,6 +223,7 @@ config SA1100_WATCHDOG
 
 config DW_WATCHDOG
 	tristate "Synopsys DesignWare watchdog"
+	depends on HAS_IOMEM
 	help
 	  Say Y here if to include support for the Synopsys DesignWare
 	  watchdog timer found in many chips.