dt-bindings: iio: imu: st_lsm6dsx: support active-low interrupts

Update st_lsm6dsx device binding with active-low interrupts support
(IRQ_TYPE_LEVEL_LOW and IRQ_TYPE_EDGE_FALLING).

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