regulator: gpio-regulator: Catch 'no states property' misuse

A selection of voltage or current values (AKA states) should always
be specified when using a GPIO regulator. If there are no switchable
states then the fixed regulators should be used instead.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>