media: platform: sti: Adopt SPDX identifier

Add SPDX identifiers to files under sti directory

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Acked-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>