staging: IIO: trigger: New Blackfin specific trigger driver iio-trig-bfin-timer

This driver allows any Blackfin system timer to be used as IIO trigger.
It supports trigger rates from 0 to 100kHz in Hz resolution.

Changes since V1:
IIO: trigger: Apply review feedback

Add comment explaining Blackfin hardware timer configurations
Fix frequency_store, don't return -EINVAL for frequency set to 0

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>