remoteproc: Add prepare and unprepare ops

remoteproc: Add prepare and unprepare ops

On some SoC architecture, it is needed to enable HW like
clock, bus, regulator, memory region... before loading
co-processor firmware.

This patch introduces prepare and unprepare ops to execute
platform specific function before firmware loading and after
stop execution.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200417002036.24359-2-s-anna@ti.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>