arm64: defconfig: Enable QTI_IOMMU_SUPPORT only on target configurations

Selecting QTI_IOMMU_SUPPORT as part of ARM_SMMU is problematic
as ARM_SMMU will be enabled in the gki_defconfig, which adds the
APIs exposed by QTI_IOMMU_SUPPORT to the ABI surface. This is
not correct, as those symbols are not part of GKI. Thus, do not
select QTI_IOMMU_SUPPORT when ARM_SMMU is enabled, but rather
require for it to be enabled explicitly.

Change-Id: I587c206dd11deac5537d8c0efef42d4a16d42c49
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>