iio: imu: inv_icm42600: make timestamp module chip independent

Move icm42600 dependent function inside the core module.
Do some headers cleanup at the same time.

Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20230606162147.79667-2-inv.git-commit@tdk.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>