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: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>