remoteproc: Introducing function rproc_attach()

Introducing function rproc_attach() to enact the same actions as
rproc_start(), but without the steps related to the handling of
a firmware image.  That way we can properly deal with scenarios
where the remoteproc core needs to attach with a remote processsor
that has been booted by another entity.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20200714195035.1426873-4-mathieu.poirier@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>