Replace dts includes with C preprocessor syntax

Using the /include/ syntax, the include was evaluated by dtc, only after running
the preprocessor, therefore the .dtsi files were not preprocessed. This patch
adds the #include syntax instead. Evaluating this and preprocessing the files
now happens in a single step, done by the C preprocessor.

Change-Id: I6d0104b6274316fc736e84973502a4d6c2c9d6e0
Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>