Rather than having the RPM SMD channels as the only child of a dummy
SMD node, switch to representing the RPM as remoteproc like all the
other remoteprocs (WCNSS, modem DSP).

This allows assigning additional subdevices to it like the MPM
interrupt-controller or rpm-master-stats.
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20230531-rpm-rproc-v3-10-a07dcdefd918@gerhold.netSigned-off-by: Bjorn Andersson <andersson@kernel.org>