iio: light: Add support for Azoteq IQS621/622 ambient light sensors

This patch adds support for the Azoteq IQS621 and IQS622 ambient light
sensors, both of which can report a four-bit representation of ambient
light intensity.

The IQS621 can additionally report illuminace directly in units of lux,
while the IQS622 can report a four-bit representation of infrared light
intensity. Furthermore, the IQS622 can report a unitless measurement of
a target's proximity to the device.

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>