powerpc/mpc83xx: add support for new SPL

This adds arch support for PPC mpc83xx to boot "minimal" (4K) SPLs
using the new infrastructure.

Existing nand_spl targets are updated to deal with the name change
from nand_init.c to spl_minimal.c (as in theory this isn't limited
to NAND anymore).

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>