powerpc/5200: Add support for the Media5200 board from Freescale

This patch adds board support for the Media5200 platform.  Changes are:
- add the media5200 device tree
- add the media5200 platform support code and cascaded interrupt controller
- add media5200 to the build targets.

Note: this patch also includes a minor tweak to the lite5200(b) target
images list to add the .dtb files to the image list.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>