video: Add a dark-grey console colour

This is useful for highlighting something with a black background, as
is needed with cedit when using a white-on-black console. Add this as
a new colour.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/video.h

@@ -181,6 +181,7 @@ enum colour_idx {
 	VID_LIGHT_MAGENTA,
 	VID_LIGHT_CYAN,
 	VID_WHITE,
+	VID_DARK_GREY,
 
 	VID_COLOUR_COUNT
 };