Adding overlay support for i.MX3 sdc.
Foreground channel only works when background is also enabled.
The foreground video mode is always the same as the background.
Also added alpha command to set the alpha value of the foreground.
Tested on a phyCORE-i.MX35.
Signed-off-by: Teresa Gámez <t.gamez@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>