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>
diff --git a/include/video/omapdss.h b/include/video/omapdss.h
index c7cfdad..31b7746 100644
--- a/include/video/omapdss.h
+++ b/include/video/omapdss.h
@@ -510,6 +510,7 @@
 		} dispc;
 
 		struct {
+			/* regn is one greater than TRM's REGN value */
 			u16 regn;
 			u16 regm;
 			u16 regm_dispc;