arm: socfpga: dts: Move to SPDX license identifiers

The socfpga dts files are copied from the Rocketboards.org repository.
In U-Boot we usually replace the full-blown license header text with
the SPDX license identifiers. Lets do this for these new dts files
as well.

I just forgot to do this while adding the DT support for socfpga.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Vince Bridgers <vbridger@altera.com>