iio: imu: inv_mpu6050: add fifo temperature data support

Add support of temperature data in fifo for all chips.
Enable unification of scan elements for icm20602.
Add macros for generating scan elements.

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