ARM: tegra: Add SMMU enabler in AHB

Add extern func, "tegra_ahb_enable_smmu()" to inform AHB that SMMU is
ready.

Signed-off-by: Hiroshi DOYU <hdoyu@nvidia.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>