dt-bindings: iio: vadc: Fix documentation of 'reg'

The documentation of Qualcomm's SPMI PMIC voltage ADC 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: Andy Gross <andy.gross@linaro.org>