dts/bindings: sensors: remove '-magn' extension from lis2mdl name

The LIS2MDL is not a combo device, but pure magnotemeter.
Hence, '-magn' extension is not adding information and can
be removed from dts compatible name as well as binding filename.

Instead specify '-i2c' or '-spi' to distinguish between the names.

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