regulator: s2mps11: Allow GPIO 0 to be used as external control on S2MPS14

GPIO 0 is a valid GPIO so allow using it as external control for
S2MPS14 regulators.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>