iommu/arm-smmu: fix programming of SMMU_CBn_TCR for stage 1

Stage-1 context banks do not have the SMMU_CBn_TCR[SL0] field since it
is only applicable to stage-2 context banks.

This patch ensures that we don't set the reserved TCR bits for stage-1
translations.

Cc: <stable@vger.kernel.org>
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>