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 <thierry.reding@gmail.com>
Cc: Benson Leung <bleung@chromium.org>
Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: linux-pwm@vger.kernel.org
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>