armv8: layerscape: Update early MMU for DDR after initialization

armv8: layerscape: Update early MMU for DDR after initialization

In early MMU table, DDR has to be mapped as device memory to avoid
speculative access. After DDR is initialized, it needs to be updated
to normal memory to allow code execution. To simplify the code,
dram_init() is moved into a common file as a weak function.

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