Add memory test feature for mpc85xx POST.

The memory test is performed after DDR initialization when U-boot stills runs
in flash and cache. On recent mpc85xx platforms, the total memory can be more
than 2GB. To cover whole memory, it needs be mapped 2GB at a time using a
sliding TLB window. After the testing, DDR is remapped with up to 2GB memory
from the lowest address as normal.

If memory test fails, DDR DIMM SPD and DDR controller registers are dumped for
further debugging.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>