drivers/sensor: stmemsc: make use of spi_dt_spec and i2c_dt_spec

Make use of the new DT facilities that introduced two new
bus structures, spi_dt_spec and i2c_dt_spec, as well as the
macros, SPI_DT_SPEC_INST_GET and I2C_DT_SPEC_INST_GET, to
retrieve info from DT.

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