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>