arm-smmu: Fix iommu group refcount leak

If a client defines the qcom,iommu-group property, and the iommu group
does not yet exist, of_get_device_group will create a new group with a
refcount of one. The following call to iommu_group_ref_get() increments
the refcount unnecessarily and causes a resource leak.

If a client defines the qcom,iommu-group property, and the iommu group
already exists, modify __bus_lookup_iommu_group() to return with
an additional refcount.

Change-Id: I5c9e62e50737ee4128a591ea2b14a0d47a1be162
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>