dt-bindings: net: renesas-ravb: Make stream buffer optional

The Stream Buffer for EtherAVB-IF (STBE) is an optional component, and
is not present on all SoCs.

Document this in the DT bindings, including a list of SoCs that do have
it.

Fixes: 785ec874 ("ravb: document R8A77970 bindings")
Fixes: f231c417 ("dt-bindings: net: renesas-ravb: Add support for R8A77995 RAVB")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>