remoteproc: Introduce prepare and unprepare for subdevices

On rare occasions a subdevice might need to prepare some hardware
resources before a remote processor is booted, and clean up some
state after it has been shut down.

One such example is the IP Accelerator found in various Qualcomm
platforms, which is accessed directly from both the modem remoteproc
and the application subsystem and requires an intricate lockstep
process when bringing the modem up and down.

Tested-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
[elder@linaro.org: minor description and comment edits]
Signed-off-by Alex Elder <elder@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>