Documentation: dt: iio: imu: st_lsm6dsx: add st,drdy-int-pin property

Add st,drdy-int-pin property to select interrupt pin of the package

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>