iio: adc: lpc32xx: Add scale feature

Until now this driver only exposed the raw value of the channels. With
this patch, the scale value is also exposed.

It depends of a regulator supply, and unlike most of the other driver, do
not having this regulator won't prevent to use the driver. The reason for
it is to allow to continue to use this driver with an old device tree. If
there is no regulator supply then the scale won't be exposed.

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