iio: adc: qcom-spmi-adc5: Add ADC5_GPIO2_100K_PU

Even though it existed in bindings for the longest time,
ADC5_GPIO2_100K_PU was never assigned in the driver. Do so.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20230627-topic-adc-v1-1-c61581abffa3@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>