Added a property to enable user space to set aspect ratio.
This patch contains declaration of the property and code to create the
property.

v2: Thierry's review comments.
	- Made aspect ratio enum generic instead of HDMI/CEA specfic
	- Removed usage of temporary aspect_ratio variable

v3: Thierry's review comments.
	- Fixed indentation

v4: Thierry's review comments.
	- Return ENOMEM when property creation fails
Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>