Staging: IIO: TRIGGER: New sysfs based trigger

This patch adds a new trigger that can be invoked by writing
the sysfs file: trigger_now. This approach can be valuable during
automated testing or in situations, where other trigger methods
are not applicable. For example no RTC or spare GPIOs.
Last but not least we can allow user space applications to produce triggers.

IIO: TRIGGER: Apply review feedback by Greg Kroah-Hartman

Changes since v1:

Add sysfs documentation.
Change license notice.
Add module alias.
Add more Kconfig help text

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>