Staging: iio: adt7316: Use device tree data to set ldac_pin

Make the driver use device tree instead of the platform data.
Hence, use devm_gpiod_get_optional function to get the data from
device tree for ldac-pin and accordingly make the needed changes
in the driver.

Signed-off-by: Shreeya Patel <shreeya.patel23498@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>