iio: accel: mma9553: add enable channel for activity

Add an enable channel for activity, so it can also
be polled independently of events or other channels.

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