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: Richard Weinberger <richard@nod.at>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>