driver/ddr: Restruct driver to allow standalone memory space

U-boot has been initializing DDR for the main memory. The presumption
is the memory stays as a big continuous block, either linear or
interleaved. This change is to support putting some DDR controllers
to separated space without counting into main memory. The standalone
memory controller could use different number of DIMM slots.

Signed-off-by: York Sun <yorksun@freescale.com>