power: reset: Add reset driver for R-Mobile platforms

Add a reset driver for Renesas R-Mobile and SH-Mobile SoCs. It registers
a restart handler to trigger a soft power-on reset through the R-Mobile
System Controller.
The priority of this restart handler is 192, to allow a watchdog driver
to use priority 128.

Note that we do not use syscon-reboot, as the HPB (Peripheral Bus
Bridge) semaphore should be acquired on systems where both the ARM and
SH core are in use. The driver can be extended later to support this,
when needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sebastian Reichel <sre@kernel.org>