armv8: fsl-layerscape: Put SMMU config code in SMMU_BASE

It is not mandatory for Layerscape SoCs to have SMMU. SoCs like
LS1012A are layerscape SoC without SMMU IP.

So put SMMU configuration code under SMMU_BASE.

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>