iio: ina2xx: provide a sysfs parameter to allow async readout of the ADCs

This can lead to repeated or skipped samples depending on the clock beat
between the capture thread and the chip sampling clock, but will also spare
reading/waiting for the Capture Ready Flag and improve the available i2c
bandwidth for reading measurements.

Output of iio_info:
...snip...
4 device-specific attributes found:
  attr 0: in_oversampling_ratio value: 4
  attr 1: in_allow_async_readout value: 0
  attr 2: integration_time_available value: 140 204 332 588 1100 2116...
  attr 3: in_sampling_frequency value: 114

Signed-off-by: Marc Titinger <mtitinger@baylibre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>