ARM: dts: aspeed: Update license headers

In b2441318 ("License cleanup: add SPDX GPL-2.0 license identifier
to files with no license") these files had the GPL-2.0 licence added
automatically. Update them to be GPL 2.0+ in line with other IBM kernel
contributions.
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>

arch/arm/boot/dts/aspeed-ast2500-evb.dts

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0+
 /dts-v1/;
 
 #include "aspeed-g5.dtsi"

arch/arm/boot/dts/aspeed-bmc-opp-palmetto.dts

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0+
 /dts-v1/;
 
 #include "aspeed-g4.dtsi"

arch/arm/boot/dts/aspeed-bmc-opp-romulus.dts

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0+
 /dts-v1/;
 
 #include "aspeed-g5.dtsi"

arch/arm/boot/dts/aspeed-g4.dtsi

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

arch/arm/boot/dts/aspeed-g5.dtsi

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