iio: cros_ec: Register to cros_ec_sensorhub when EC supports FIFO

When EC supports FIFO, each IIO device registers a callback, to put
samples in the buffer when they arrives from the FIFO.

When no FIFO, the user space app needs to call trigger_new, or better
register a high precision timer.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>