staging:iio:trigger handle name attr in core, remove old alloc and register any control_attrs via struct device

As the majority of triggers don't actually have any other control_attrs lets use the fact
that struct device has a groups element when we do need to have these attributes registered.
A vargs function is used to cut down on lots of building strings in every single driver
just in order to pass them into the allocate.

Also iio_allocate_trigger_named -> iio_allocate_trigger as there is no
unamed version any more, so that is now just confusing.

Blackfin tested and fixed by Michael Hennerich.

V2: Elements from Michael Hennerich's patches for the ade7758
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>