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

* 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>;

		};

	};