remoteproc: Add Renesas rcar driver

Renesas Gen3 platform includes a Cortex-r7 processor.

Both: the application cores (A5x) and the realtime core (CR7)
share access to the RAM and devices with the same address map,
so device addresses are equal to the Linux physical addresses.

In order to initialize this remote processor we need to:
- power on the realtime core
- put the firmware in a RAM area
- set the boot address for this firmware (reset vector)
- Deassert the reset

This initial driver allows to start and stop the Cortex R7
processor.

Signed-off-by: Julien Massot <julien.massot@iot.bzh>
Link: https://lore.kernel.org/r/20211207165829.195537-3-julien.massot@iot.bzh
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>