Documentation: dt-bindings: add exynos-chipid binding information

Exynos SoC's DT files are using Chipid device nodes, but it's binding
information is missing. This patch adds exynos-chipid binding information.
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>

Documentation/devicetree/bindings/arm/samsung/exynos-chipid.txt

+SAMSUNG Exynos SoCs Chipid driver.
+
+Required properties:
+- compatible : Should at least contain "samsung,exynos4210-chipid".
+
+- reg: offset and length of the register set
+
+Example:
+	chipid@10000000 {
+		compatible = "samsung,exynos4210-chipid";
+		reg = <0x10000000 0x100>;
+	};