mailbox: Fix dependencies for !HAS_IOMEM archs

Not every arch has io memory.
So, unbreak the build by fixing the dependencies.
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/Kconfig

@@ -81,6 +81,7 @@ config STI_MBOX
 config MAILBOX_TEST
 	tristate "Mailbox Test Client"
 	depends on OF
+	depends on HAS_IOMEM
 	help
 	  Test client to help with testing new Controller driver
 	  implementations.