ARM: shmobile: r8a7779: Rename DU device in clock lookups list

The DU device will be called rcar-du-r8a7779. Rename the clock lookup
entry accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>