devicetree: bindings: document SYSRAM binding

Samsung Exynos SYSRAM for SMP bringup:

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Samsung SMP-capable Exynos SoCs use part of the SYSRAM for the bringup

of the secondary cores. Once the core gets powered up it executes the

code that is residing at some specific location of the SYSRAM.

Therefore reserved section sub-nodes have to be added to the mmio-sram

declaration. These nodes are of two types depending upon secure or

non-secure execution environment.

Required sub-node properties:

- compatible : depending upon boot mode, should be

		"samsung,exynos4210-sysram" : for Secure SYSRAM

		"samsung,exynos4210-sysram-ns" : for Non-secure SYSRAM

The rest of the properties should follow the generic mmio-sram discription

found in ../../misc/sysram.txt

Example:

	sysram@02020000 {

		compatible = "mmio-sram";

		reg = <0x02020000 0x54000>;

		#address-cells = <1>;

		#size-cells = <1>;

		ranges = <0 0x02020000 0x54000>;

		smp-sysram@0 {

			compatible = "samsung,exynos4210-sysram";

			reg = <0x0 0x1000>;

		};

		smp-sysram@53000 {

			compatible = "samsung,exynos4210-sysram-ns";

			reg = <0x53000 0x1000>;

		};

	};