iio: dummy: Demonstrate the usage of new channel types

Adds support for the new channel types in the dummy driver:
  * a new channel IIO_ACTIVITY
  * two state transition events (running and walking)
  * a new channel IIO_STEPS and support for reading and writing
    pedometer step counter
  * step detect event
  * a new IIO_CHAN_INFO_CALIBHEIGHT mask bit for reading and writing
    user's height.

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>