iommu/arm-smmu: Use dev_iommu_fwspec_get() to retrieve IOMMU fwspec

Use the dev_iommu_fwspec_get() accessor, instead of directly
accessing the device structure for the IOMMU fwspec, as well
as remove an unnecessary forward declaration.

Change-Id: Id09202bd468bddbf8bdf194513fda34b59756fba
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>