samples: sensor: lsm6dsl

This commit provides sample application for sensor lsm6dsl.
This sample has been tested on both 96b_argonkey board,
where lsm6dsl is connect to the micro thru SPI bus, and on
disco_l475_iot1 board, where instead it is connected to I2C bus.

Signed-off-by: Armando Visconti <armando.visconti@st.com>