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>

Documentation/devicetree/bindings/net/renesas,ravb.txt

@@ -27,7 +27,11 @@ Required properties:
 	SoC-specific version corresponding to the platform first followed by
 	the generic version.
 
-- reg: offset and length of (1) the register block and (2) the stream buffer.
+- reg: Offset and length of (1) the register block and (2) the stream buffer.
+       The region for the register block is mandatory.
+       The region for the stream buffer is optional, as it is only present on
+       R-Car Gen2 and RZ/G1 SoCs, and on R-Car H3 (R8A7795), M3-W (R8A7796),
+       and M3-N (R8A77965).
 - interrupts: A list of interrupt-specifiers, one for each entry in
 	      interrupt-names.
 	      If interrupt-names is not present, an interrupt specifier