ARM: socfpga: dts: convert to preprocessor includes

Convert all socfpga DT files to the dtc preprocessor include syntax.
This allows to include header files in the devicetrees like other
SoC-types already do.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>