Revert "iommu/arm-smmu-v3: Implement shutdown method"

This reverts commit 7aa8619a ("iommu/arm-smmu-v3: Implement shutdown
method").

Disabling the SMMU during shutdown is currently causing issues because
there are SMMU clients who don't currently support shutdown and there is
currently no guarantee that shutdown will be called on SMMU clients before
shutdown is called on the SMMU.

We don't need to support kexec so revert SMMU shutdown support until it
can be cleanly handled.

Change-Id: Ia042ea29e6432d14d42e3536f82775161a934b9c
Signed-off-by: Liam Mark <lmark@codeaurora.org>