dt-bindings: iio: add missing reset-gpios constrain

The Documentation/devicetree/bindings/gpio/gpio-consumer-common.yaml
schema does not enforce number of reset GPIOs, thus each device binding
must do it.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231005083650.92222-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>