staging: add missing SPDX lines to Makefile files

There are a few remaining drivers/staging/*/Makefile files that do not
have SPDX identifiers in them.  Add the correct GPL-2.0 identifier to
them to make scanning tools happy.

Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>