dt-bindings: net: ravb: Add support for r8a77990 SoC

Add documentation for r8a77990 compatible string to renesas ravb device
tree bindings documentation.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>