arm: dts: socfpga*.dts*: use SPDX-License-Identifier

Follow the recent trend for the license description.

This is also in an effort to fully sync the devicetrees with U-Boot.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>