powerpc: ppc4xx: Change from OF_SEPARATE to OF_EMBED

This is necessary, as ppc4xx has the reset vector located at the end
of the U-Boot image. This needs to be flashed to the end of the NOR
flash. Adding the dtb to the main U-Boot image will break booting
on ppc4xx. This patch now embeds the dtb in the U-Boot image instead.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>