ARM: shmobile: koelsch-reference: Enable CMT0 in device tree

No more device needs to be added from platform code when booting the
reference platform, remove the now empty r8a7791_add_dt_devices()
function completely.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>