iio: imu: st_lsm6dsx: Decouple sensor ODR from FIFO batch data rate

iio: imu: st_lsm6dsx: Decouple sensor ODR from FIFO batch data rate

The rate at which accelerometer or gyroscope sensor samples are fed
to the hardware FIFO (batch data rate, or BDR) does not have to
coincide with the sensor sampling frequency (output data rate, or
ODR); the only requirement is for the BDR to not be greater than
the ODR. Having a BDR lower than the ODR is useful in cases where
an application requires a high sampling rate for accurate detection
of motion events (e.g. wakeup events), but wants to read sensor
sample values from the hardware FIFO at a lower data rate (e.g. to
minimize the amount of I2C or SPI traffic and the rate of periodic
interrupts).
To support the above use case, add a sampling_frequency sysfs
attribute to the buffer directory of st_lsm6dsx IIO devices, which
controls the BDR for a given sensor independently from the "main"
sampling_frequency attribute (which controls the ODR); introduce a
new `hwfifo_odr_mHz` field in struct st_lsm6dsx_sensor to keep
track of the current BDR value, and use this field instead of the
`odr` field in the code that deals with the FIFO data rate. In the
sensor hub driver, make the hwfifo_odr_mHz value always mirror the
odr value, since there is no separate configuration setting to
control the BDR for data produced by the sensor hub functionality.
For backwards compatibility, set the buffer frequency equal to the
main frequency whenever the latter is updated via sysfs; if
userspace wants a different buffer frequency, it has to write to
the relevant sysfs attribute after any writes to the main frequency
attribute.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>