iommu/arm-smmu: fix aperture_end calculation

qcom,iommu-geometry property sets start and size. Fix geometry.aperture_end
calcuation to add start to size.

Fixes: e98585fe ("iommu/arm-smmu: add qcom,iommu-geometry DT property")
Change-Id: Iebc8e6ad46e391b5f208ec75f140aea04ba9beb7
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>