powerpc/mpc85xx: Temporary fix for spin table backward compatibility

Once u-boot sets the spin table to cache-enabled memory, old kernel which
uses cache-inhibit mapping without coherence will not work properly. We
use this temporary fix until kernel has updated its spin table code.
For now this fix is activated by default. To disable this fix for new
kernel, set environmental variable "spin_table_compat=no". After kernel
has updated spin table code, this default shall be changed.

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