arm: dts: Add new "generic" 66AK2Gx device tree file.

With U-boot runtime board detect for DTB selection a "default" dtb needs
to be created. This will be used temporarily until the "proper" dtb is
selected.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>