[media] media-device: export the entity function via new ioctl

Now that entities have a main function, expose it via
MEDIA_IOC_G_TOPOLOGY ioctl.

Please notice that some entities may have secundary functions.
Such use case will be addressed later, when we add support for the
Media Controller properties.

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

include/uapi/linux/media.h

@@ -276,7 +276,8 @@ struct media_links_enum {
 struct media_v2_entity {
 	__u32 id;
 	char name[64];		/* FIXME: move to a property? (RFC says so) */
-	__u16 reserved[14];
+	__u32 function;		/* Main function of the entity */
+	__u16 reserved[12];
 };
 
 /* Should match the specific fields at media_intf_devnode */