Update and fix NXP copyright

The copyright should be: Copyright <year range> NXP
There is no ( C ).
Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 274c4f426f414b0ea3d205f582ab479a9f291879)

iMX8M/mkimage_imx8.c

 /*
  * Copyright (C) 2016 Freescale Semiconductor, Inc.
+ * Copyright 2017-2018 NXP
  *
  * SPDX-License-Identifier:	GPL-2.0+
  * derived from u-boot's mkimage utility

src/imx8qm.c

 /*
- * Copyright (C) 2017 NXP
+ * Copyright 2017 NXP
  *
  * SPDX-License-Identifier:     GPL-2.0+
  * derived from u-boot's mkimage utility

src/imx8qx.c

 /*
- * Copyright (C) 2017 NXP
+ * Copyright 2017 NXP
  *
  * SPDX-License-Identifier:     GPL-2.0+
  * derived from u-boot's mkimage utility

src/mkimage_common.h

 /*
- * Copyright (C) 2017 NXP
+ * Copyright 2017 NXP
  *
  * SPDX-License-Identifier:     GPL-2.0+
  * derived from u-boot's mkimage utility

src/mkimage_imx8.c

 /*
  * Copyright (C) 2016 Freescale Semiconductor, Inc.
  *
- * Copyright (C) 2017 NXP
+ * Copyright 2017 NXP
  *
  * SPDX-License-Identifier:	GPL-2.0+
  * derived from u-boot's mkimage utility