drivers/sensor: lis2ds12: Add multi-instance support

This commit aligns lis2ds12 sensor driver to latest multi
instance sensor driver model.

In particular it makes use of the stmemsc common routines
and move ctx handler inside struct config, so that the
bus_init routines can be totally avoided.

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