dt-bindings: timer: renesas,rz-mtu3: Document RZ/{G2UL,Five} SoCs

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 <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/timer/renesas,rz-mtu3.yaml

···
   compatible:
     items:
       - enum:
+          - renesas,r9a07g043-mtu3  # RZ/{G2UL,Five}
           - renesas,r9a07g044-mtu3  # RZ/G2{L,LC}
           - renesas,r9a07g054-mtu3  # RZ/V2L
       - const: renesas,rz-mtu3