soc: renesas: Add Renesas R-Car MFIS driver

Renesas R-Car MFIS offers multiple features but most importantly
mailboxes and hwspinlocks. Because they share a common register space
and a common register unprotection mechanism, a single driver was chosen
to handle all dependencies. (MFD and auxiliary bus have been tried as
well, but they failed because of circular dependencies.)

In this first step, the driver implements common register access and a
mailbox controller. hwspinlock support will be added incrementally, once
the subsystem allows out-of-directory drivers.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20260402112709.13002-3-wsa+renesas@sang-engineering.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

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