armv8: fsl-layerscape: add i/d-cache enable function to enable_caches

This function assume that the d-cache and MMU has been enabled earlier,
so it just created MMU table in main memory. But the assumption is not
always correct, for example, the early setup is done in EL3, while
enable_caches() is called when the PE has turned into another EL.

Define the function mmu_setup() for fsl-layerscape to cover the weak
one.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>