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>

Documentation/devicetree/bindings/sram/renesas,smp-sram.txt

+* Renesas SMP SRAM
+
+Renesas R-Car Gen2 and RZ/G1 SoCs need a small piece of SRAM for the jump stub
+for secondary CPU bringup and CPU hotplug.
+This memory is reserved by adding a child node to a "mmio-sram" node, cfr.
+Documentation/devicetree/bindings/sram/sram.txt.
+
+Required child node properties:
+  - compatible: Must be "renesas,smp-sram",
+  - reg: Address and length of the reserved SRAM.
+    The full physical (bus) address must be aligned to a 256 KiB boundary.
+
+
+Example:
+
+	icram1:	sram@e63c0000 {
+		compatible = "mmio-sram";
+		reg = <0 0xe63c0000 0 0x1000>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges = <0 0 0xe63c0000 0x1000>;
+
+		smp-sram@0 {
+			compatible = "renesas,smp-sram";
+			reg = <0 0x10>;
+		};
+	};