iommu/tegra: smmu: Change SMMU's dependency on ARCH_TEGRA

Theoretically TEGRA_IOMMU_SMMU depends on ARCH_TEGRA_3x_SOC and
ARCH_TEGRA_114_SOC only. This patch allows a Tegra20 only kernel to
enable SMMU(Tegra20 doesn't have a SMMU), which could avoid editing
this Kconfig entry every time we add a new chip later.

Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>