iio: imu: st_lsm6dsx: add odr calibration feature

On LSM6DSO/LSM6DSR/LSM6DSOX/ASM330LHH and ISH330DHCX
devices it is possible to trim the hardware timestamp
resolution through the FREQ_FINE[7:0] bits of the
INTERNAL_FREQ_FINE register, which contains the difference
in percentage of the effective ODR (and timestamp rate)
with respect to the typical value.

The formula for calculating the effective ODR reported
in the application notes has been linearized to the first
order to simplify the calculation (pls. see note on source
code).

This change may be useful in the outcome of CTS
tests regarding the SingleSensorTests and the
SensorTest#testSensorTimeStamps for high ODRs

Signed-off-by: Mario Tesi <mario.tesi@st.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>