staging: fsl-mc: Add SPDX license identifiers

The fsl-mc bus and dpio driver files use a combination of GPL-2.0 and
GPL-2.0+ OR BSD-3-Clause licenses. Add SPDX tags and delete the full
license text, keeping the existing license for each file. Add GPL-2.0
for Kconfig / Makefile, where not present.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Reviewed-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>