ARM: dts: rcar-gen2: Convert to new DU DT bindings

The DU DT bindings have been updated to drop the reg-names property.
Update the r8a7792 and r8a7794 device trees accordingly.

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