The user trigger is driven by read and write. It can support one-shot
devices (those that produce or consume data only when user-space read
or writes) and streaming device, with their own internal timings.  It
is, in some way, a transparent trigger, connecting buffer and device
directly.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>