Don't try to compile shmobile-iommu outside of ARM

Commit 7d02c4d6 ("iommu/shmobile: Enable the driver on all ARM
platforms") completely brokenly enabled the shmobile-iommu driver under
COMPILE_TEST.

It's bogus, because it won't compile anywhere else than ARM, since it
tries to include <asm/dma-iommu.h>, which is very much ARM-only.

So remove the bogus COMPILE_TEST dependency, which just causes
allmodconfig to fail on non-ARM platforms.

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