iommu: shmobile: Enable driver compilation with COMPILE_TEST

This helps increasing build testing coverage.

The driver doesn't compile on non-ARM platforms due to usage of the ARM
DMA IOMMU API, restrict compilation to ARM.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: iommu@lists.linux-foundation.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Joerg Roedel <joro@8bytes.org>