arm64: dts: renesas: r8a77990: Fix SPDX license identifier style

According to Documentation/process/license-rules.rst, SPDX license
identifiers in DTS files should use C++ style comments.

Fixes: f37a7767 ("arm64: dts: renesas: Add Renesas R8A77990 SoC support")
Fixes: 77049191 ("arm64: dts: renesas: Add Renesas Ebisu board support")
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/r8a77990-ebisu.dts

-/* SPDX-License-Identifier: GPL-2.0 */
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Device Tree Source for the ebisu board
  *

arch/arm64/boot/dts/renesas/r8a77990.dtsi

-/* SPDX-License-Identifier: GPL-2.0 */
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Device Tree Source for the R-Car E3 (R8A77990) SoC
  *