Add MTU3a binding documentation for Renesas RZ/{G2UL,Five} SoCs.

MTU3a block is identical to one found on RZ/G2L, so no driver changes are
required. The fallback compatible string "renesas,rz-mtu3" will be used
on RZ/{G2UL,Five}.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230727081848.100834-4-biju.das.jz@bp.renesas.com