iio: add modifiers for linear acceleration

Add IIO_MOD_LINEAR_X, IIO_MOD_LINEAR_Y and IIO_MOD_LINEAR_Z modifiers to te
IIO core, which is preparatory for adding the Bosch BNO055 IMU driver.

Bosch BNO055 IMU can report raw accelerations (among x, y and z axis) as
well as the so called "linear accelerations" (again, among x, y and z axis)
which is basically the acceleration after subtracting gravity and for which
those new modifiers are for.

Signed-off-by: Andrea Merello <andrea.merello@iit.it>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220907132205.28021-2-andrea.merello@iit.it


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>