mailbox: altera: Add dependency on HAS_IOMEM

Not all architectures have io memory.

Fixes:
drivers/built-in.o: In function `altera_mbox_probe':
mailbox-altera.c:(.text+0x409fd2): undefined reference to `devm_ioremap_resource'
Signed-off-by: NRichard Weinberger <richard@nod.at>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/Kconfig

@@ -56,6 +56,7 @@ config PCC
 
 config ALTERA_MBOX
 	tristate "Altera Mailbox"
+	depends on HAS_IOMEM
 	help
 	  An implementation of the Altera Mailbox soft core. It is used
 	  to send message between processors. Say Y here if you want to use the