arm64: dts: ti: Makefile: Clarify GPL-2.0 as GPL-2.0-only

SPDX identifier GPL-2.0 has been deprecated since license list version
3.0. Use GPL-2.0-only to be specific.
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240122145539.194512-17-nm@ti.comSigned-off-by: Vignesh Raghavendra <vigneshr@ti.com>

arch/arm64/boot/dts/ti/Makefile

-# SPDX-License-Identifier: GPL-2.0
+# SPDX-License-Identifier: GPL-2.0-only
 #
 # Make file to build device tree binaries for boards based on
 # Texas Instruments Inc processors