mpc83xx: Add bank configuration to FSL spd_sdram.c

The routine assumed 4 bank SDRAMs, enhance to configure for 4 or 8
bank SDRAMs.

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>
Acked-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>