soc: renesas: rcar-rst: Add support to set rproc boot address

R-Car Gen3 SoC series has a realtime processor, the boot
address of this processor can be set thanks to CR7BAR register
of the reset module.

Export this function so that it's possible to set the boot
address from a remoteproc driver.

Also drop the __initdata qualifier on rcar_rst_base,
since we will use this address later than init time.

Signed-off-by: Julien Massot <julien.massot@iot.bzh>
Link: https://lore.kernel.org/r/20211022122101.66998-1-julien.massot@iot.bzh
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>