qcom: subsystem_restart: Introduce a callback to free memory

Certain subsystems may require memory associated with
a subsystem to be freed after a memory dump has been
taken. Previously, this was expected to be done in the
shutdown callback. Add a new callback that implementations
can take advantage of to free memory when they see fit,
either in shutdown or in the new callback.

Change-Id: I28e5ab2d7be6937e15b9c129dcabc7963462249e
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>