iio: light: rpr0521 triggered buffer

Set up and use triggered buffer if there is irq defined for device in
device tree. Trigger producer triggers from rpr0521 drdy interrupt line.
Trigger consumer reads rpr0521 data to scan buffer.
Depends on previous commits of _scale and _offset.
Signed-off-by: Mikko Koivunen <mikko.koivunen@fi.rohmeurope.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>