iommu/shmobile: Depend on ARCH_SHMOBILE

Commit 78a2e12f ("iommu: shmobile: Enable driver compilation with
COMPILE_TEST") added an optional dependency on SH_MOBILE. But that
Kconfig symbol doesn't exist. It seems ARCH_SHMOBILE was intended. Use
that.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>