dt-bindings: iio: humidity: hts221: support active-low interrupts

Update hts221 device binding with active-low interrupts support
(IRQ_TYPE_LEVEL_LOW and IRQ_TYPE_EDGE_FALLING).

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