src: add copyright comment headers

src/main/scala/uk/ac/soton/ecs/can/core/DataMemory.scala

+// SPDX-FileCopyrightText: 2021 Minyong Li <ml10g20@soton.ac.uk>
+// SPDX-License-Identifier: CERN-OHL-W-2.0
+
 package uk.ac.soton.ecs.can.core
 
 import chisel3._

src/main/scala/uk/ac/soton/ecs/can/core/ProgramMemory.scala

+// SPDX-FileCopyrightText: 2021 Minyong Li <ml10g20@soton.ac.uk>
+// SPDX-License-Identifier: CERN-OHL-W-2.0
+
 package uk.ac.soton.ecs.can.core
 
 import chisel3._

src/main/scala/uk/ac/soton/ecs/can/core/QuarterRound.scala

+// SPDX-FileCopyrightText: 2021 Minyong Li <ml10g20@soton.ac.uk>
+// SPDX-License-Identifier: CERN-OHL-W-2.0
+
 package uk.ac.soton.ecs.can.core
 
 import chisel3._

src/test/scala/uk/ac/soton/ecs/can/core/DataMemoryTest.scala

+// SPDX-FileCopyrightText: 2021 Minyong Li <ml10g20@soton.ac.uk>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package uk.ac.soton.ecs.can.core
 
 import org.scalatest._

src/test/scala/uk/ac/soton/ecs/can/core/ProgramMemoryTest.scala

+// SPDX-FileCopyrightText: 2021 Minyong Li <ml10g20@soton.ac.uk>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package uk.ac.soton.ecs.can.core
 
 import org.scalatest._

src/test/scala/uk/ac/soton/ecs/can/core/QuarterRoundTest.scala

+// SPDX-FileCopyrightText: 2021 Minyong Li <ml10g20@soton.ac.uk>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 package uk.ac.soton.ecs.can.core
 
 import org.scalatest._