Convert mpc7448hpc2 to CONFIG_OF_LIBFDT

This was configured to use the deprecated CONFIG_OF_FLAT_TREE, change
to CONFIG_OF_LIBFDT.

WARNING: This conversion is untested because I do not have a board to
test it on.

NOTE: The FDT blob (DTS) must have an /aliases/ethernet0 and (optionally)
/aliases/ethernet1 property for the ethernet to work.

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>