iommu/arm-smmu: fix compilation issue when !CONFIG_ARM_AMBA

If !CONFIG_ARM_AMBA, we shouldn't try to register ourselves with the
amba_bustype.

Change-Id: I0dc34cd5f3ccf88141f4fe738036eb4af5aa4c4f
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: d123cf82d339c5cc4ffe2a481e0caa23a501d4ac
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>