remoteproc: Add support for runtime PM

Call pm_runtime_get_sync() before the firmware is loaded, and
pm_runtime_put() after the remote processor has been stopped.

Even though the remoteproc device has no PM callbacks, this allows the
parent device's PM callbacks to be properly called.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20200515104340.10473-3-paul@crapouillou.net
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>