drm/bridge/synopsys: dsi: Adopt SPDX identifiers

Add SPDX identifiers to the Synopsys DesignWare MIPI DSI
host controller driver.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Acked-by: Philippe Ombredanne <pombredanne@nexB.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208145805.24762-1-philippe.cornu@st.com