ethernet: renesas: convert to SPDX identifiers

This patch updates license to use SPDX-License-Identifier
instead of verbose license text.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/Kconfig

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
 #
 # Renesas device configuration
 #