drivers/sensor: lis2mdl: Add multi-instance support

Add multi-instance support and make use of the stmemsc i2c/spi
read/write routine that has been introduced to simplify the ST
sensor drivers code.

Moreover, move spi-full-duplex property from Kconfig inside Device
Tree, so that each LIS2MDL instance can be configured selectively
in accordance to how it is used in h/w.

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