iio: imu: add support to lsm6dsx driver

Add support to STM LSM6DS3-LSM6DSM 6-axis (acc + gyro) Mems sensor

http://www.st.com/resource/en/datasheet/lsm6ds3.pdf
http://www.st.com/resource/en/datasheet/lsm6dsm.pdf



- continuous mode support
- i2c support
- spi support
- sw fifo mode support
- supported devices: lsm6ds3, lsm6dsm

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>