sf: Add GPL-2.0+ SPDX-License-Identifier for missing ones

Added GPL-2.0+ SPDX-License-Identifier for missed sf
source files.

Signed-off-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Signed-off-by: Bo Shen <voice.shen@atmel.com>
5 files changed