Staging: IIO: Periodic timer based trigger

The original posting of this driver led to a discussion in
which it was commented that a better system was needed
for dealing with the many possible periodic interrupt
sources available on some SoCs.  Unfortunately that is
a big task and as far as I know, no-one has taken it
on as yet.  So in the meantime this driver is still
in here.

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