mpc83xx: convert to using do_fixup_*()

convert to using simpler mpc85xx style fdt update code; streamline by
eliminating macros OF_SOC, OF_CPU, etc. which allows us to rm
the old school FLAT_TREE code from 83xx (since the sbc8349 was just
converted over to using libfdt).

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