dt-bindings: mailbox: apple,mailbox: Add t6020 compatible
The mailbox hardware remains unchanged on M2 Pro/Max/Ultra SoCs so just
add its per-SoC compatible.
Reviewed-by: Neal Gompa <neal@gompa.dev>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Janne Grunau <j@jannau.net>