iio: add adjd_s311 I2C digital color sensor driver

iio: add adjd_s311 I2C digital color sensor driver

sensor has 4 channels (10-bit each, R/G/B and clear), sensitivity
and gain is controlled in the driver by ext_info integration_time
and CHAN_INFO_HARDWAREGAIN

driver supports triggered buffer and IIO_CHAN_INFO_RAW to get the
sensor data

v5: address comments by Jonathan Cameron
* use macro for channel declaration
* get timestamp right before measurement
* cleanups

v4: address comments by Lars-Peter Clausen
* make sure trigger handler is exited with iio_trigger_notify_done()
  and IRQ_HANDLED
* kfree()/kalloc() -> krealloc()

v3:
* fix warnings

v2: address comments by Lars-Peter Clausen
* buffer allocation now in update_scan_mode instead of in trigger
  handler
* simplify trigger code (assume active_scan_mask is not empty, use
  for_each_set_bit, use iio_push_to_buffer)
* reorder entry in Makefile and Kconfig
* fix remove

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>