soc: renesas: rcar-mfis: Add R-Car V4H/V4M support

soc: renesas: rcar-mfis: Add R-Car V4H/V4M support

The above SoCs have a weird register layout for the mailbox registers.
So, encapsulate register offset calculation in a per-SoC callback. Other
than that, only a separate config struct and compatibles are needed.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20260519075620.4128-3-wsa+renesas@sang-engineering.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>