iio: light: veml6070: add support for integration time

The integration time of the veml6070 depends on an external resistor
(called Rset in the datasheet) and the value configured in the IT
field of the command register, whose supported values are 1/2x, 1x,
2x and 4x.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241031-veml6070-integration-time-v4-2-c66da6788256@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>