staging:iio: Add infrastructure for irq_chip based triggers

V3: Get rid of separate interrupt pool. This is well handled
    by irq_get_descs and irq_free_descs.  Two functions I simply
    wasn't aware of previously.  Thus the allocation for a given
    trigger is now handled by core code rather than us reinventing
    the wheel.

V2: Stop silly name duplication.
    Move pool handling to industrialio-trigger as that is the only user.
    Changed over to using irq_modify_status rather than the arm
    specific set_irq_flags as per Thomas Gleixner's suggestion.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>