iommu/arm-smmu: add and use definitions for the TTBCR2.SEP field

Currently we're repurposing the definitions used for the TTBCR2.PASize
field for TTBCR2.SEP since they conveniently have the same values.
However, this might look like a bug at first glance to the casual
passer-by, appearing that we're using the wrong bit definitions for the
field.  For example, we're using TTBCR2_ADDR_32 to indicate that the
Sign Extension Bit should live at 31 (not 32 as the name of the macro
might imply).  Reduce cumulative human cognitive load by adding some
definitions specifically for the SEP field.

Change-Id: Ia406951499453e2badca42a1cc4cdbc566af4dab
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>