remoteproc: Add new attach() remoteproc operation

remoteproc: Add new attach() remoteproc operation

Add an new attach() operation in order to properly deal with
scenarios where the remoteproc core needs to attach to a
remote processor 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/20200714195035.1426873-3-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>