remoteproc: imx_rproc: support i.MX7ULP

i.MX7ULP A7 core runs under control of M4 core, M4 core starts by ROM
and powers most services used by A7 core, so A7 core has no power to
start and stop M4 core. And the M4 core's state is default RPROC_DETACHED
and remoteproc framework not able to stop the M4 core.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/1620274123-1461-8-git-send-email-peng.fan@oss.nxp.com


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