iio: imu: inv_icm42600: add temperature sensor support

Add temperature channel in gyroscope and accelerometer devices.

Temperature is available in full 16 bits resolution when reading
register and in low 8 bits resolution in the FIFO. Return full
precision raw temperature with corresponding scale and offset.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>