drm: rcar-du: Enable compilation on ARM64

The R8A7795 SoC is ARM64-based and include a DU. Enable driver
compilation on ARM64.

Signed-off-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>