powerpc/fsl: Move Freescale device tree files into fsl folder

It makes no sense that some Freescale device tree files are in fsl
directory while some others not. This patch move Freescale device tree
files into fsl folder. To do that the following two steps are made:
- Move Freescale device tree files into fsl folder.
- Update the include path in these files from "fsl/*.dtsi" to "*.dtsi".

Please add "fsl/" prefix when you make dtb using Makefile.

Signed-off-by: Jia Hongtao <hongtao.jia@freescale.com>
[scottwood: fixed cuImage rule]
Signed-off-by: Scott Wood <scottwood@freescale.com>