iio: bmc150: introduce bmc150_accel_trigger

Add a separate structure for triggers and add the infrastructure to
support an arbitrary number of triggers. Each trigger is associated
with an interrupt and has an enabled/disabled state.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>