iio: Add channel type for attention

Add a new channel type representing if the user's attention state to the
the system. This usually means if the user is looking at the screen or
not.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://patch.msgid.link/20241101-hpd-v3-3-e9c80b7c7164@chromium.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>