dt-bindings: mfd: qcom,spmi-pmic: Use generic node name "gpio"

GPIO controller nodes are named by convention just "gpio", not "gpios".

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220908080938.29199-3-krzysztof.kozlowski@linaro.org