powerpc/mpc8xxx: Add auto select bank interleaving mode

Based on populated DIMMs, automatically select from cs0_cs1_cs2_cs3 or
cs0_cs1 interleaving, or non-interleaving if not available.

Fix the message of interleaving disabled if controller interleaving
is enabled but DIMMs don't support it.

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