iio: light: ROHM BU27034 Ambient Light Sensor

ROHM BU27034 is an ambient light sensor with 3 channels and 3 photo diodes
capable of detecting a very wide range of illuminance. Typical application
is adjusting LCD and backlight power of TVs and mobile phones.

Add initial  support for the ROHM BU27034 ambient light sensor.

NOTE:
	- Driver exposes 4 channels. One IIO_LIGHT channel providing the
	  calculated lux values based on measured data from diodes #0 and
	  #1. In addition, 3 IIO_INTENSITY channels are emitting the raw
	  register data from all diodes for more intense user-space
	  computations.
	- Sensor has GAIN values that can be adjusted from 1x to 4096x.
	- Sensor has adjustible measurement times of 5, 55, 100, 200 and
	  400 mS. Driver does not support 5 mS which has special
	  limitations.
	- Driver exposes standard 'scale' adjustment which is
	  implemented by:
		1) Trying to adjust only the GAIN
		2) If GAIN adjustment alone can't provide requested
		   scale, adjusting both the time and the gain is
		   attempted.
	- Driver exposes writable INT_TIME property that can be used
	  for adjusting the measurement time. Time adjustment will also
	  cause the driver to try to adjust the GAIN so that the
	  overall scale is kept as close to the original as possible.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/2a7efb6f335da5526fbe34b95137c5e45db5c5d3.1680263956.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>