treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 250

Based on 1 normalized pattern(s):

  gpl v2 can be found in copying

extracted by the scancode license scanner the SPDX license identifier

  GPL-2.0-only

has been chosen to replace the boilerplate/reference in 20 file(s).

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>