mfd: google,cros-ec: add DT bindings for a baseboard's switch device

This is for ChromeOS tablets which have a 'cros_cbas' switch device
in the "Whiskers" base board. This device can be instantiated only by
device tree on ARM platforms. ChromeOS EC doesn't provide a way to
probe the device.
Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/google,cros-ec.yaml

@@ -117,6 +117,22 @@ properties:
       - "#address-cells"
       - "#size-cells"
 
+  cbas:
+    type: object
+
+    description:
+      This device is used to signal when a detachable base is attached
+      to a Chrome OS tablet. This device cannot be detected at runtime.
+
+    properties:
+      compatible:
+        const: google,cros-cbas
+
+    required:
+      - compatible
+
+    additionalProperties: false
+
 patternProperties:
   "^i2c-tunnel[0-9]*$":
     type: object
@@ -187,6 +203,10 @@ examples:
             proximity {
                     compatible = "google,cros-ec-mkbp-proximity";
             };
+
+            cbas {
+                compatible = "google,cros-cbas";
+            };
         };
     };