iommu: msm: simplify address discovery in secure register dump code

Currently in the secure register dump code, we try to determine the
register in question by doing some reverse arithmetic on the full
address that TZ returns and matching that against known register
offsets. However, support was recently added for storing the base
physical address of the Iommu in `struct msm_iommu_drvdata'. Simplify
the code by calculating the offset of the registers being returned by TZ
by subtracting their values from the base address of the Iommu.

Change-Id: Icd6e4a35a48b808b0523ab4971cb9fb5b00125f5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>