mux: add SPDX identifiers to all mux source files

Remove all free-text license texts.

This is done on a quest to remove the 700+ different ways that files in
the kernel describe the GPL license text.

No copyright headers or other non-license-description text was removed.

Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>