MPC8xx: Fix libfdt support introduced in commit 77ff7b74

fdt.c: In function 'ft_cpu_setup':
fdt.c:33: warning: implicit declaration of function 'do_fixup_by_prop_u32'
fdt.c:39: warning: implicit declaration of function 'do_fixup_by_compat_u32'
fdt.c:43: warning: implicit declaration of function 'fdt_fixup_ethernet'
fdt.c:45: warning: implicit declaration of function 'fdt_fixup_memory'

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>