iio: imu: st_lsm6dsx: inline output acc/gyro output regs

Remove output register definition and inline register value since
they are used only for iio channel definition. This is a preliminary
patch to add support for LSM9DS1 sensor to st_lsm6dsx driver

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>