OMAP: DSS2: DSI: Add comment about regn

regn divider is one greater than the REGN divider in TRM. Add a comment
to point this out.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

include/video/omapdss.h

@@ -510,6 +510,7 @@ struct omap_dss_device {
 		} dispc;
 
 		struct {
+			/* regn is one greater than TRM's REGN value */
 			u16 regn;
 			u16 regm;
 			u16 regm_dispc;