iio: gyro: st_gyro_spi: add OF capability to st_gyro_spi

Add device tree support for L3G4200D, LSM330D, LSM330DL, LSM330DLC,
L3GD20, L3GD20H, L3G4IS, LSM330, LSM9DS0 gyroscope sensors

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>