remoteproc: Add new RPROC_ATTACHED state

Add a new RPROC_ATTACHED state to take into account scenarios
where the remoteproc core needs to attach to a remote processor
that is booted by another entity.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20210312162453.1234145-4-mathieu.poirier@linaro.org


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