iio: add hardware triggered operating mode

Devices, like stm32 timer, could be triggered by hardware events which
are not buffer or software events. However it could be necessary to
validate the triggers like it is done for buffer or event triggered modes.
This patch add a new INDIO_HARDWARE_TRIGGERED operating mode for this
kind of devices and allow this mode to register trigger consumer.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>