Documentation/drm: Describing aspect ratio property

Updated drm documentation to include desscription of aspect ratio property.

v2: Updated aspect ratio specific documentation on top of the HTML table created.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Cc: Sagar Kamble <sagar.a.kamble@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sagar Kamble <sagar.a.kamble@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>