cosmetic: aspeed: Modify for SPDX-License

Modify SPDX-License for furture patch warning

Signed-off-by: Ryan Chen <ryan_chen@aspeedtech.com>
Reviewed-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>

arch/arm/dts/ast2500-u-boot.dtsi

+// SPDX-License-Identifier: GPL-2.0
 #include <dt-bindings/clock/aspeed-clock.h>
 #include <dt-bindings/reset/ast2500-reset.h>
 

include/dt-bindings/clock/aspeed-clock.h

-/* SPDX-License-Identifier: GPL-2.0+ */
+// SPDX-License-Identifier: GPL-2.0
 
 #define ASPEED_CLK_GATE_ECLK		0
 #define ASPEED_CLK_GATE_GCLK		1