staging: sm750fb: add proper SPDX identifier to driver

staging: sm750fb: add proper SPDX identifier to driver

By default all files without license information are under the default
license of the kernel, which is GPL version 2.  Because of this, add the
GPL-2.0 identifier to the sm750fb driver which did not have any license
identifiers in it at all.

Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>