Remove the deprecated CONFIG_OF_FLAT_TREE

The following troika of patches switch the final two boards (mpc7448hpc2 and stxxst) from using CONFIG_OF_FLAT_TREE to CONFIG_OF_LIBFDT and then remove support for CONFIG_OF_FLAT_TREE.

Since I don't have access to the boards I could not test them. If someone can test the conversions and provide patches, I would appreciate it.

For the stxxtc board, I don't see how it uses CONFIG_OF_* at all, leading me to believe that the FDT support for that board has bit-rotted away already. Again, patches are welcome.

Best regards,
gvb

Sorry for the noise, I didn't do a full job, working on the rest of the job.

gvb