dt-bindings: iio: adc: move lpc32xx-adc out of staging

The drivers has been moved out since
0097e20e ("staging:iio:adc:lpc32xx Move out of staging.") in v4.10,
so let's align the binding documentation.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>