cbus: retu: give it a context structure

This is pretty much a cleanup patch just
adding a context structure for Retu, to avoid
all the globals it had.

Note that this breaks retu-user.c due to moving
the lock around, but that retu-user.c has to
go anyway as it's completely non-standard way
of accessing Retu children.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>