devicetree: bindings: document SYSRAM binding

Add SRAM binding documentation.
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
[kgene.kim@samsung.com: changed name SRAM to SYSRAM]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>

Documentation/devicetree/bindings/arm/exynos/smp-sysram.txt

+Samsung Exynos SYSRAM for SMP bringup:
+------------------------------------
+
+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>;
+		};
+	};