powerpc/mpc85xx:Disable Debug TLB entry for non-minimal SPL

CONFIG_SPL_BUILD creates debug TLB entry, so disable it before init_tlbs.

CONFIG_SPL_INIT_MINIMAL never creates any debug TLB entry, so no need
of disable_tlb().

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>