iommu: msm: remove unused field from struct dump_regs_tbl_entry

The field `offset' is not used and is potentially confusing since its
name is similar to its sibling `reg_offset' field.

Change-Id: Iad00a57399dd3de07fcf1e1908666c11ce08a82d
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>