remoteproc: sysmon: Add notifications for events

Add notification for other stages of remoteproc boot and shutdown. This
includes adding callback functions for the prepare and unprepare events,
and fleshing out the callback function for start.

Acked-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Siddharth Gupta <sidgup@codeaurora.org>
Link: https://lore.kernel.org/r/1586389003-26675-3-git-send-email-sidgup@codeaurora.org


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