OMAP: OMAPFB: Adding help for FB_OMAP_LCD_VGA option

CONFIG_FB_OMAP_LCD_VGA option is present in drivers/video/omap
There is no explaination about what this flag does.
Lets add information about it.

FB_OMAP_LCD_VGA flag sets resolution of display to VGA (640 X 480).
The default resolution of 3430 LDP is 320 X 240.

Signed-off-by: Mayuresh Janorkar <mayur@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>