mpc83xx: mpc8360emds: rework LBC SDRAM setup

Currently 64M of LBC SDRAM are mapped at 0xF0000000 which makes
it difficult to use (b/c then the memory is discontinuous and
there is quite big memory hole between the DDR/SDRAM regions).

This patch reworks LBC SDRAM setup so that now we dynamically
place the LBC SDRAM near the DDR (or at 0x0 if there isn't any
DDR memory).

With this patch we're able to:

- Boot without external DDR memory;
- Use most "DDR + SDRAM" setups without need to support for
  sparse/discontinuous memory model in the software.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>