ravb: add device tree support for r8a779[123]

Simply document new compatibility strings.
As a previous patch adds a generic R-Car Gen2 compatibility string
there appears to be no need for a driver updates.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>