iommu/arm-smmu: use "no-sign" SEP value

Currently, we program the TCR2.SEP to be equal to the input address
size.  This makes it impossible to use the top bit of the input address
space since that causes TTBR1 to be selected, which we don't program or
use.  Since we don't even use sign-extended addresses, fix this by
always using the "no-sign" value for TCR2.SEP as specified by the ARM
SMMUv2 spec.

Change-Id: I9155b62fe59c66185c6b9fba7f8bba8798b45785
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>