iio: accel: kxcjk1013: allow using an external trigger

In its present state, the driver mandates that its buffer only be
triggered by one of the device's own triggers (data ready or any
motion).  This is not always desirable, for example because the
interrupt pins may not be wired in.

Patch the driver to be able to accept using an external trigger, such as
one based on hrtimer.  When using such a trigger, we need to ensure that
the device is powered on when the buffer is started.  We do that by
setting setup_ops for the buffer.

Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>