iio: event_monitor: Add support for new channel types

We have the following testing scenario:

$ insmod iio_dummy_evgen.ko
$ insmod iio_dummy.ko

./iio_event_monitor /dev/iio:device0
Event: time: 1412786467971335337, type: activity(running), channel: 0,
evtype: thresh, direction: rising
Event: time: 1412786530792974091, type: activity(walking), channel: 0,
evtype: thresh, direction: falling
Event: time: 1412764319184761765, type: steps, channel: 0, evtype: instance

$ echo 1 > /sys/bus/iio/devices/iio_evgen/poke_ev0
$ echo 2 > /sys/bus/iio/devices/iio_evgen/poke_ev0
$ echo 3 > /sys/bus/iio/devices/iio_evgen/poke_ev0

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>