[media] uapi/media.h: Fix entity namespace

Now that interfaces got created, we need to fix the entity
namespace.

So, let's create a consistent new namespace and add backward
compatibility macros to keep the old namespace preserved.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>