dt-bindings: sram: Document renesas,smp-sram

Document reserved SRAM for the SMP jump stub on Renesas R-Car Gen2 and
RZ/G1 SoCs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>