dt-bindings: thermal: qcom-spmi-temp-alarm: Fix documentation of 'reg'

The documentation claims that the 'reg' property consists of two values,
the SPMI address and the length of the controller's registers. However
the SPMI bus to which it is added specifies "#size-cells = <0>;". Remove
the controller register length from the documentation of the field and the
example.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>