iommu/io-pgtable-fast: fix sign extension position

The introduction of support for ttbr1 wrongly caused the
fastmap stage 1 input address sign extension position to
be set to bit 31.
This was causing valid IOVAs to page fault.
Set the sign extension position back to its previous value.

Change-Id: I95ca88f8c033dd79006eff8dfea85007d091f869
Signed-off-by: Liam Mark <lmark@codeaurora.org>