dt: don't use ARCH_CPU_DTS

Now that we assume dtc supports the -i option, we don't need to use
ARCH_CPU_DTS in *.dts{,i}; we simply specify the include filename
directly, and dtc will find it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>