dt-bindings: pwm: Fix cros-ec-pwm example dtc 'reg' warning

The example for the CrOS EC PWM is incomplete and now generates a dtc
warning:

Documentation/devicetree/bindings/pwm/google,cros-ec-pwm.example.dts:17.11-23.11:
Warning (unit_address_vs_reg): /example-0/cros-ec@0: node has a unit name, but no reg or ranges property

Fixing this results in more warnings as a parent spi node is needed as
well.

Cc: Thierry Reding <[e-mail]>
Cc: Benson Leung <[e-mail]>
Cc: Enric Balletbo i Serra <[e-mail]>
Cc: Guenter Roeck <[e-mail]>
Cc: [e-mail]
Acked-by: Uwe Kleine-König <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

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

···
 
 examples:
   - |
-    cros-ec@0 {
-        compatible = "google,cros-ec-spi";
-        cros_ec_pwm: ec-pwm {
-            compatible = "google,cros-ec-pwm";
-            #pwm-cells = <1>;
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        cros-ec@0 {
+            compatible = "google,cros-ec-spi";
+            reg = <0>;
+
+            cros_ec_pwm: ec-pwm {
+                compatible = "google,cros-ec-pwm";
+                #pwm-cells = <1>;
+            };
         };
     };