metag: *.dts: include using preprocessor

Include *.dtsi files from *.dts using the preprocessor to set a good
example for future device tree files. Files included in the old way
don't get pre-processed.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: devicetree-discuss@lists.ozlabs.org