dt-bindings: mailbox: apple,mailbox: Add generic and t6000 compatibles

dt-bindings: mailbox: apple,mailbox: Add generic and t6000 compatibles

Much as we've done with other blocks, let's introduce generic
compatibles so drivers can bind to those and still work with future
SoCs, as long as the hardware remains the same. Also go ahead and add
compatibles for the new t600x SoCs (we group those as t6000).

Note that no DTs instantiate devices with this binding yet.

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>