iio: acpi_als: Add trigger support

As some firmware does not notify on illuminance changes, add a
trigger to be able to query light via software (sysfs-trigger or
hrtrigger).
Add a hardware trigger set as the default trigger to maintain backward
compatibility.

Check iio_info reports the sensor as buffer capable:
  iio:device0: acpi-als (buffer capable)

To test, check we can get data on demand on an Intel based chromebook:

  IIO_DEV="iio:device0"
  echo 1 > iio_sysfs_trigger/add_trigger
  cat trigger2/name > ${IIO_DEV}/trigger/current_trigger
  for i in ${IIO_DEV}/scan_elements/*_en ${IIO_DEV}/buffer/enable ; do
    echo 1 > $i
  done
  od -x /dev/${IIO_DEV} &
  echo 1 > trigger2/trigger_now

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210317074012.2336454-4-gwendal@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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