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>
diff --git a/arch/arm/dts/ast2500-u-boot.dtsi b/arch/arm/dts/ast2500-u-boot.dtsi
index 29b08f1..51a5244 100644
--- a/arch/arm/dts/ast2500-u-boot.dtsi
+++ b/arch/arm/dts/ast2500-u-boot.dtsi
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 #include <dt-bindings/clock/aspeed-clock.h>
 #include <dt-bindings/reset/ast2500-reset.h>