doc: Add a binding for sysinfo

Add a simple binding file for this, so that it is clear what this binding
directory is for.
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

doc/device-tree-bindings/sysinfo/sysinfo.txt

+Sysinfo
+=======
+
+This provides capabilities to access information about a board/system, for
+use by drivers.
+
+Required properties:
+
+  - compatible: any suitable string where the driver is in the UCLASS_SYSINFO
+      class
+
+See also smbios.txt
+
+
+Example
+
+sysinfo {
+	compatible = "sandbox,sysinfo-sandbox";
+};