staging:iio:triggers: rename iio-trig-gpio to iio-trig-interrupt

Also change all internal naming appropriately.
This trigger is no longer just for gpio provided interrupts so
change the naming to reflect this.  Also drop some now missleading
left over comments.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>